<p>We do not know whether or not we live in a simulation in which our capricious simulation overlords have conspired to hide evidence that we are growing larger. Discounting that possibility, science says we are not growing larger.</p>
<p>The expansion of the universe is something that happens at very large distance scales. At moderate distance scales gravitation overwhelms expansion, at least for now, and will continue to do so for trillions of years in the future. At even smaller scales (e.g., us), electromagnetic interactions overwhelm gravity.</p>
<p>We are not expanding, nor is the Earth, nor is the solar system, nor is the galaxy. Our galaxy is on a headlong course to collide with the Andromeda Galaxy in about five billion years due to gravitation. The expansion of the universe is something that happens on an even larger scale.</p>

CC BY-SA 4.0
2023-02-03T21:11:00.477Z
This does not answer the question: how do we know? It just tells what we know, as if "take it or leave it".
Roland Pihlakas
CC BY-SA 4.0
2023-02-05T13:08:19.867Z
@RolandPihlakas It does answer the question as far as I'm concerned. One either accepts that science is more or less correct, which clearly says we're not getting bigger for reasons elaborated in the answer, or one goes to the woo side of pseudoscience that says anything goes. That woo side is impossible to disprove. What if there's some malevolent super-being who has made key observations we make somewhat fictional? What if we're living in a simulation made by capricious super-beings who decided to toy with the simulated minds of their simulated under-beings (e.g., us)?
David Hammen
CC BY-SA 4.0
2023-02-05T17:33:37.523Z
@ David Hammen The question *explicitly* asked "how" do we know, not "what" we know. I other words, OP wanted to understand. This response does not even attempt to answer the "how" question. It could be even argued that believing the "science" without understanding it is closer to pseudoscience than the desire to understand would be. How is one going to learn science if the responses are like "just trust in the science"? I am surprised that you dismissed my concern.
Roland Pihlakas
CC BY-SA 4.0
2023-02-06T12:33:12.840Z
@RolandPihlakas The answer is indeed just trust in the science. As my edit says, we do not and cannot know whether we are simulated beings. I prefer to think we are not. Accepted science says the expansion is a very large scale phenomenon while at small scales (groups of galaxies are "small scale"), the interactions we know of completely overwhelm the expansion of space.
David Hammen
CC BY-SA 4.0
2023-02-01T20:34:55.170Z
2023-02-06T12:29:07.363Z
2023-02-06T12:29:07.363Z
David Hammen
<p><strong>Matter contraction:</strong> If everything got twice as big then you are right that rulers etc. would not be able to measure it. However, some physical constants are expressed (partly) in meters, and it would appear that these constants had halved (because the meter stick has doubled).</p>
<p>For example my distance from the centre of the Earth will have doubled, so gravity would get weaker. We would need a new value for <span class="math-container">$G$</span>. Light would look like it was going slower, so <span class="math-container">$c$</span> would seem smaller.</p>
<p><strong>Space expansion:</strong> Interestingly, the expansion of the universe is actually connected to the opposite situation. We see that the distance between stars is twice as many meter-sticks than we infer it used to be. The standard explanation (quite reasonably!) is that the distance has indeed doubled. But we could offer the explanation that the distance remained the same, but the meter stick has halved in length. Matter of a fixed size in an expanding universe looks kind of the same as a universe of a fixed size with <em>shrinking</em> matter.</p>
<p>But the shrinking has to come with physical constants changing to compensate, otherwise we would notice.</p>

CC BY-SA 4.0
2023-02-02T21:55:06.570Z
wouldn't $c$ remain the same due to  general/special relativity?
an inconspicuous semicolon
CC BY-SA 4.0
2023-02-03T02:11:01.757Z
@blankettripod $c$ (and other physical constants) would remain the same. But the numerical value of $c$, expressed in $m/s$, could change if the length of the meter changed.
Carmeister
CC BY-SA 4.0
2023-02-03T11:33:48.503Z
oh i see, that makes more sense, so if 1 meter became 2, $c$ would become $1.5*10^8$ but would still be the same speed, right?
an inconspicuous semicolon
CC BY-SA 4.0
2023-02-03T14:49:41.183Z
@blankettripod Yes. But in order to match our current models, either $c$ would have to change, or we'd have to speed up / slow down as we changed size. (Positing that we're growing / shrinking, _and_ all the physical constants are changing in exactly the way that'd prevent us from noticing this, is a more complicated theory than just assuming we're not.)
wizzwizz4
CC BY-SA 4.0
2023-02-03T21:17:29.667Z
I have heard that light speed c is nowadays defined as a fixed value and meter is now defined as variable - as whatever distance that light passes during one second. Not the way around. Is that true? If so, then the length of a meter would change without c value changing. Do I get that right?
Roland Pihlakas
CC BY-SA 4.0
2023-02-05T13:50:51.150Z
@RolandPihlakas That new physics-based definition of the International System of Units (SI for short; the acronym is French) assumes the speed of light is constant. There is absolutely no reason to disbelieve this. Given ever improving observations that the speed of light is indeed constant (and that various other physical quantities are constant), the SI community decided about four years ago to use those constants of nature as the basis for a complete revamping of the SI base units.
David Hammen
CC BY-SA 4.0
2023-02-05T17:38:28.587Z
@David Hammen The objective of the question is not whether I am disbelieving that speed of light is constant. Instead I am asking what are the consequences of this sort of setup. Is a meter potentially changing its length? Just as with your other response and comment, again I am surprised that you similar theme here "There is absolutely no reason to disbelieve this" - focusing on forcing my beliefs instead of responding to the content of the question. Why?
Roland Pihlakas
CC BY-SA 4.0
2023-05-26T19:19:08.630Z
Interestingly enough, I once did a calculation on how much energy would be radiated if all matter in the universe were shrinking, based on the idea that mass would decrease concordantly.  It came out to a pretty reasonable value (small enough that we probably wouldn't detect it locally) as well as comparable in total to the estimated dark energy content of the universe.
Michael
CC BY-SA 4.0
2023-02-02T16:09:37.743Z
2023-02-02T16:09:37.743Z
Dast
<p>Beyond the other answers here, we <em>can</em> measure an upper limit on how fast we're growing.  For example, imagine that everything doubled in size.  You're right that our rulers would double in size and everything would seems the same size from that perspective.</p>
<p>However, since everything is twice as big, light now needs twice as long to travel the distances that it used to.  Every network engineer using fibreoptic cables suddenly sees their latency double.  Every GPS unit fails, as the calculation is partially dependent on using the time it takes the signal to travel from the satellite to the device.  Radios would fail as the frequency controlled by the electronics would no longer match the wavelength optimized by the antenna (though this wouldn't be quite as prfound if it was truly an exact doubling).</p>
<p>To be more succinct, the speed of light is <em>defined</em> as 299,792,458 meters per second.  The length of the meter and the duration of the second are linked by this fundamental definition.  If something comes along and changes the meter, then the second is redefined to match.  If the duration of a second was changing, it would be pretty noticeable.</p>

CC BY-SA 4.0
2023-02-02T21:15:52.707Z
Perhaps light gets faster? Perhaps we will feel the growth, it depends how linear it is
Rabbi Kaii
CC BY-SA 4.0
2023-02-03T09:05:11.390Z
"If the duration of a second was changing, it would be pretty noticeable." - why wouldn't our perception of it change to match the actual change?
Karl Knechtel
CC BY-SA 4.0
2023-02-02T10:43:48.020Z
2023-02-02T10:51:58.993Z
2023-02-02T10:51:58.993Z
rprospero
<p><strong>Space is not expanding,</strong> at least not in the way that you are thinking. Expanding space is just a convention that simplifies some of the mathematics in the context of cosmology. The contents of the universe are expanding -- moving apart in a uniform way -- and this process is conveniently described using a coordinate system in which space is taken to expand. But that is just a convention. Expanding space does not have local physical consequences.</p>
<p>This means that the expansion of space does not cause objects to expand. Not only that, <em>it doesn't even supply an expansion force that has to be counteracted by gravity or electromagnetism</em>, as is often mistakenly believed.</p>
<p><strong>Why then is the universe expanding, if not because of the expansion of space?</strong> Imagine throwing a ball up into the air at escape velocity. After the ball departs, there is no continued impetus for separation; indeed gravity is pulling the ball back. And yet the ball continues to move away from the Earth due its initial motion alone. It's similar with the universe. Cosmic expansion is a consequence of motion in the initial conditions.</p>
<h3>Notes</h3>
<p>There is one physical sense in which space might be expanding. If the universe is finite, then its total volume (measured on comoving surfaces) grows. This does not affect local dynamics, though.</p>
<p>Also, dark energy supplies an expansion force. However, this is more clearly viewed as <em>the cause of</em> accelerated expansion than a consequence of it. Also, the force itself is most clearly interpreted as just gravity, since it arises from the equations of general relativity.</p>
<h3>Further reading</h3>
<p>I also include some choice quotes.</p>
<ul>
<li><a href="https://arxiv.org/abs/0809.4573" rel="noreferrer">A diatribe on expanding space.</a> &quot;This analysis demonstrates that there is no local effect on particle dynamics from the global expansion of the universe: the tendency to separate is a kinematic initial condition, and once this is removed, all memory of the expansion is lost.&quot;</li>
<li><a href="https://arxiv.org/abs/0808.1081" rel="noreferrer">The kinematic origin of the cosmological redshift.</a> &quot;The tendency [of the Solar System] to expand due to the stretching of space is nonexistent, not merely negligible.&quot;</li>
<li><a href="https://arxiv.org/abs/1605.08634" rel="noreferrer">On The Relativity of Redshifts: Does Space Really &quot;Expand&quot;?</a> &quot;But if you assume that expanding space is something physical, something like a river carrying distant observers along as the universe expands,
the consequence of this when considering the motions of objects in the universe will lead to radically incorrect results.&quot;</li>
</ul>

CC BY-SA 4.0
2023-02-03T21:23:43.010Z
Since when does the space not expand? Was big bang expansion of space or the particles moving away from each other? I have heard so far that big bang was expansion of space, so did something change later?
Roland Pihlakas
CC BY-SA 4.0
2023-02-04T09:22:49.783Z
@RolandPihlakas As [Steven Weinberg said](https://www.newscientist.com/article/mg13818693-600/), "*Cosmologists sometimes talk about expanding space – but they should know better.*" To be clear, cosmology is a mathematical science, and everyone agrees on the mathematics. There is no question about the mathematical, and hence physical, description of an expanding universe. The only question is how we translate the mathematics into words or pictures. And "expanding space" is a translation that leads to a lot of misconceptions, like the one that led to this question.
Sten
CC BY-SA 4.0
2023-02-10T14:20:10.627Z
Dear @Sten, The statement that "The tendency of the Solar System to expand due to the stretching of space is nonexistent" is incorrect. https://www.sciencedaily.com/releases/2012/10/121009161103.htm
Michael_1812
CC BY-SA 4.0
2023-02-11T01:21:35.767Z
@Michael_1812 That paper's conclusion would imply that cosmic expansion affects local dynamics in the [Milne universe](https://en.wikipedia.org/wiki/Milne_model), which is impossible because the Milne universe is just re-parametrized Minkowski spacetime. The paper is wrong.
Sten
CC BY-SA 4.0
2023-02-11T14:13:44.233Z
@Sten The linear FLRW is mathematically equiv to Milne, which is mathematically equiv to a global Minkowski spacetime. However,  the global Minkowski time T is not the local Minkowski time, and is not directly observable. We measure the local Minkowski time, (the proper time tau). In Milne, photons have a constant velocity in the global Minkowski spacetime. So, the tau measured by our clocks and the T along light geodesics are not the same. Kopeikin proves that the difference is measurable if our local space is expanding. His result reproduces the Pioneer anomaly, both in magnitude and sign
Michael_1812
CC BY-SA 4.0
2023-02-11T15:02:09.423Z
@Michael_1812 Are you suggesting that changing the way you parametrize spacetime changes the physical behavior of clocks?
Sten
CC BY-SA 4.0
2023-02-11T16:28:06.073Z
@Sten On the contrary, I emphasise that there is a difference between the proper time \tau and the coordinate time T. This difference is exploited in Kopeikin's work: his paper is about kinematics of clocks (timelike geodesics) and light particles (null geodesics) in two different metrics - one is globally flat and another one has a conformal factor depending on the time tau in front of the spatial metric. The Milne coordinates and the local coordinates are not identical - they are connected by a non-linear transformation.
Michael_1812
CC BY-SA 4.0
2023-02-11T16:33:46.897Z
@Michael_1812 Milne coordinates don't matter. They are just coordinates. If the article by S. Kopeikin is correct, then the influence of cosmic expansion is expected *in special relativity*, and that influence could be of any imaginable magnitude, since Minkowski spacetime can be parametrized with Milne coordinates with any expansion rate.
Sten
CC BY-SA 4.0
2023-02-11T17:20:41.647Z
@Sten We measure the proper time \tau. Also, please be mindful that global expansion exists in all viable cosmological models, which models look the same in the linear order of approximation with respect to the Hubble constant. Within this approximation, it is impossible to distinguish (mathematically) the Milne model from the FLRW one. To do so, one would have to extend his calculations to the second Hubble order, which Kopeikin did not do, because H^2 terms are too small to be measurable in the Pioneer experiment.
Michael_1812
CC BY-SA 4.0
2023-05-26T14:58:40.563Z
This answer's cited creates a distinction between vacuum energy and the hubble constant/space expansion, who then say "vacuum energy makes things move apart, but that isn't space expansion".   That is rather ahistorical; vacuum energy was created as a mechanism to explain space expansion.  Saying "the thing that explains X means X does not exist" seems, well, fundamentally wrong.
Yakk - Adam Nevraumont
CC BY-SA 4.0
2023-05-26T14:59:43.057Z
"In the case of vacuum-dominated models, then, the repulsive effects of vacuum energy cause all pairs of particles to separate at large times, whatever their initial kinematics; this behaviour could perhaps legitimately be called ‘expanding space’. Nevertheless, the effect stems from the clear physical cause of vacuum repulsion, and there is no new physical influence that arises purely from the fact that the universe expands. The earlier examples have proved that ‘expanding space’ is in general a dangerously flawed way of thinking about an expanding universe." arXiv:0809.4573 [astro-ph]
Yakk - Adam Nevraumont
CC BY-SA 4.0
2023-05-26T15:22:58.817Z
@Yakk-AdamNevraumont The distinction comes from the mathematics. It's possible to interpret dark energy's repulsion as a consequence of "stretching of space", but that interpretation is (1) only valid for one particular coordinate system and (2) only valid if dark energy is truly a cosmological constant (we don't know that). [General covariance](https://en.wikipedia.org/wiki/General_covariance) is a central principle of general relativity, and the "stretching of space" interpretation of the action of dark energy does not admit a covariant description.
Sten
CC BY-SA 4.0
2023-05-26T15:24:01.527Z
@Yakk-AdamNevraumont Also, dark energy was *not* created to explain cosmic expansion! Cosmic expansion would happen with or without dark energy. Dark energy was only devised to explain a particular feature in the expansion history.
Sten
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2023-05-26T15:57:15.660Z
@Sten how do you explain a redshift of z=5?
Lawnmower Man
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2023-05-26T16:11:22.110Z
@LawnmowerMan What's special about $z=5$? Is this a concern about things receding "faster than light"? The answer to such concerns is that those recession rates are not relative velocities. If you were to define the recession rate of your friend in a rocket traveling away from you at near light speed in the same way that we define the recession rate of a cosmologically distant object, you would also conclude that your friend is receding much faster than light.
Sten
CC BY-SA 4.0
2023-05-26T16:32:35.960Z
In fact, there's a special case of the expanding universe solution (the [FLRW metric](https://en.wikipedia.org/wiki/FLRW)) called the [Milne model](https://en.wikipedia.org/wiki/Milne_model) (mentioned in earlier comments). It's special because it turns out to be exactly equivalent to (a portion of) the flat, static spacetime of special relativity. In spite of this, the model includes features people normally attribute to expanding space, like faster-than-light recession rates. The Milne model is one of the clearest demonstrations that "expanding space" is just a coordinate choice.
Sten
CC BY-SA 4.0
2023-05-26T18:40:47.387Z
@Sten it also requires zero energy density and negative curvature, both of which are incompatible with cosmological observations. Doesn't this put you in the crank corner of physics?
Lawnmower Man
CC BY-SA 4.0
2023-05-26T19:09:44.150Z
@LawnmowerMan What is this strawman? It's an illustrative example, showing how you can faster than light recession with no concept of expanding space.
Sten
CC BY-SA 4.0
2023-05-26T21:10:32.017Z
@Sten ok, then please explain how the observable universe is > 20 gly across but only 13 gy old. You have to reject BB/inflation or size estimates or you have FTL galaxies. Which do you pick?
Lawnmower Man
CC BY-SA 4.0
2023-05-27T00:39:56.663Z
@LawnmowerMan It's essentially time dilation. The key issue is that there is no objective definition of "now" for observers in different places. In cosmology, we choose to define "now" for a distant observer to mean "the same amount of time has elapsed since the Big Bang". But observers receding at relativistic speeds are time dilated. If an observer is receding from us at a relativistic speed, they could cover a distance much greater than 13 billion light years in an "as measured by them" (proper) time span of only 13 billion years.
Sten
CC BY-SA 4.0
2023-05-27T00:51:18.343Z
That's not an exact picture, since (for realistic cosmologies, i.e. not the Milne model) spacetime is globally curved. In such spacetimes, you can't even uniquely define a relative velocity (and hence time dilation factor) for a distant object, so it's more difficult to give an intuitive account for what's happening. Nevertheless, it should remain qualitatively clear that relativistic considerations can explain the observable universe being larger than the age of the universe.
Sten
CC BY-SA 4.0
2023-05-27T19:01:26.353Z
@Sten I think your argument is disingenuous. ΛCDM is the best-supported model and does explicitly include expansion of space. I think the strongest statement you can make is that a Milne-style cosmology might be compatible with observations, if only you can find all the missing antimatter. But pretending that it is the "true description" of reality while ΛCDM is a poorly described alternative is not the least bit justifiable.
Lawnmower Man
CC BY-SA 4.0
2023-05-27T19:35:48.263Z
@LawnmowerMan Regarding "ΛCDM is the best-supported model and does explicitly include expansion of space", the whole point is that this is false. It doesn't matter if you are considering ΛCDM, Milne, de Sitter, Einstein-de Sitter, whatever. The basic fact is that there is no such thing as an objective notion of "expansion of space" within general relativity. Spacetime simply doesn't include properties like dilation and motion. The only objective physical property of a point in spacetime is its (tensor) curvature.
Sten
CC BY-SA 4.0
2023-05-27T19:42:22.390Z
@LawnmowerMan The only reason I brought in Milne is that it is easier to develop an intuition for. It's not special otherwise.
Sten
CC BY-SA 4.0
2023-02-02T20:40:19.797Z
2023-02-02T20:40:19.797Z
Sten
<p>If the expansion affected things at the scale of stars and galaxies, we would see galaxies get smaller the further away they are.  Since that is not the case, we can deduce that the expansion doesn't happen at those scales.</p>

CC BY-SA 4.0
2023-02-03T21:21:14.780Z
I think you meant to say, if the expansion affected galaxies then they would get bigger...
Roland Pihlakas
CC BY-SA 4.0
2023-02-03T23:30:20.103Z
well another way to say that is that if everything was getting bigger at a constant rate, things that were closer to us would _appear_ to get bigger faster because apparent size has an inverse relationship to distance, which ends up being the same thing
avigil
CC BY-SA 4.0
2023-02-03T23:38:39.013Z
@RolandPihlakas they would get smaller the further away they were.  Due to the light travel time, we would be seeing them at a time when they had expanded less.
Greg Miller
CC BY-SA 4.0
2023-05-28T08:27:37.063Z
Distant galaxies *are* smaller on average (but not for this reason).
ProfRob
CC BY-SA 4.0
2023-02-02T02:53:42.350Z
2023-02-02T02:53:42.350Z
Greg Miller
<p>In a certain sense, we <em>are</em> getting bigger -- where &quot;we&quot; means: the solar system. I am talking about a physical effect, which is so feeble that it took astronomers dozens of years to measure it reliably. This is the so-called <a href="https://en.wikipedia.org/wiki/Pioneer_anomaly" rel="nofollow noreferrer">Pioneer anomaly</a>.</p>
<p>It turned out that the two Pioneer spacecraft launched in the early 70s have been decelerating anomalously as they were departing the solar system.</p>
<p>Although the topic is still being discussed, the cosmological expansion seems to be the likeliest reason for this anomaly. See, e.g., this <a href="https://www.sciencedaily.com/releases/2012/10/121009161103.htm" rel="nofollow noreferrer">press release</a> and a reference therein.</p>

CC BY-SA 4.0
2023-02-10T08:51:43.550Z
The consensus is that the Pioneer anomaly was caused by thermal emissions. The paper by Kopeikin mentioned in that press release (available non-paywalled [here](https://www.researchgate.net/publication/233967292_Celestial_dynamics_and_astrometry_in_expanding_universe)) is completely wrong and it's pretty shocking to me that it was accepted by Physical Review D. It's the same error as in the paper of Cooperstock et al. mentioned in [this answer](//physics.stackexchange.com/a/601323): he's calculating the gravitational effect of nonexistent matter.
benrg
CC BY-SA 4.0
2023-02-10T14:32:11.607Z
@benrg The work by Kopeikin was accepted by Phys Rev D for two reasons. First, it underwent *very* thorough refereeing. Second, Kopeikin is one of the world leading experts on GR. Last, and by no means least, there is no established consensus on the Pioneer anomaly.
Michael_1812
CC BY-SA 4.0
2023-02-16T23:20:56.280Z
@Michael_1812 that's interesting; one can calculate the thrust due to recoil from infrared photons radiated from one side of the spacecraft (the hot side of course) and one can know that this must happen. **In other words, *it would be a huge puzzle if this deviation was not seen!*** Related in Space SE: [Did New Horizons also demonstrate the "Pioneer Anomaly"?](https://space.stackexchange.com/q/15075/12102) (with images from simulation) and [Could radiated heat propel space-craft in outer space?](https://space.stackexchange.com/q/3591/12102)
uhoh
CC BY-SA 4.0
2023-02-16T23:22:35.600Z
@Michael_1812 and [Is the speed of the Voyager probes accurately described by Newton's or Einstein's theories?](https://space.stackexchange.com/q/14146/12102) (yes) So for "there is no established consensus" to be true, I think that you have to include flat-Earthers and conspiracy theorists in your pool of opinions.
uhoh
CC BY-SA 4.0
2023-02-04T00:25:10.783Z
2023-02-04T00:25:10.783Z
Michael_1812
<p>If everything is getting bigger at scale, so that there is no possible way to measure it, than we can't measure it. But if there is no way to measure it, there is no influence on anything. And if it has no influence, it doesn't matter, if it does.</p>

CC BY-SA 4.0
2023-02-02T09:53:34.660Z
2023-02-02T09:53:34.660Z
Christian
<p>In a closed universe, after expanding, the universe contracts.  Locally, we are in a closed region called the Local Group, and the Andromeda galaxy is approaching.  Now are we expanding to explain the collapse of the Local Group, or are we shrinking to explain large scale expansion?  (We are close to stationary with respect to the Virgo Supercluster, just to make it more problematic). Clearly, neither shrinking nor expanding would work to explain all of this.</p>
<p>GR just says that space is subjected to curvature from mass-energy, and free falling motion is always along a geodesic. If you start with particles all moving away from each other, they may continue to do so for a while, but if they have mass then the curvature of space-time slows them down. If the local density is high enough then this material is destined to fall together. But, if there are centrifugal forces or electrical forces, as in the solid Earth, to oppose the fall, then the motion can be stopped.</p>

CC BY-SA 4.0
2023-02-18T20:25:03.107Z
2023-02-19T05:05:22.380Z
2023-02-19T05:05:22.380Z
eshaya
<p>Alright so I've been thinking a lot about how the universe expands and I've always wondered if we're getting bigger as well. Since everything would be getting larger at an equal rate (tools of measurements, atoms), would it even be possible to discern if we were getting bigger or not?</p>

<galaxy><universe><expansion>
How do we know we're not getting bigger?
Landon
# How do we know we're not getting bigger?

(Asked by: username_0 on 2023-02-01)

Alright so I've been thinking a lot about how the universe expands and I've always wondered if we're getting bigger as well. Since everything would be getting larger at an equal rate (tools of measurements, atoms), would it even be possible to discern if we were getting bigger or not?

**Tags:** galaxy, universe, expansion

---

## 8 answers

### Answer by username_3 on 2023-02-02. Score: 17

**Matter contraction:** If everything got twice as big then you are right that rulers etc. would not be able to measure it. However, some physical constants are expressed (partly) in meters, and it would appear that these constants had halved (because the meter stick has doubled).

For example my distance from the centre of the Earth will have doubled, so gravity would get weaker. We would need a new value for $G$. Light would look like it was going slower, so $c$ would seem smaller.

**Space expansion:** Interestingly, the expansion of the universe is actually connected to the opposite situation. We see that the distance between stars is twice as many meter-sticks than we infer it used to be. The standard explanation (quite reasonably!) is that the distance has indeed doubled. But we could offer the explanation that the distance remained the same, but the meter stick has halved in length. Matter of a fixed size in an expanding universe looks kind of the same as a universe of a fixed size with *shrinking* matter.

But the shrinking has to come with physical constants changing to compensate, otherwise we would notice.

> **Comments:**
> **username_4**: wouldn't $c$ remain the same due to  general/special relativity?  
> **username_5**: @blankettripod $c$ (and other physical constants) would remain the same. But the numerical value of $c$, expressed in $m/s$, could change if the length of the meter changed.  
> **username_4**: oh i see, that makes more sense, so if 1 meter became 2, $c$ would become $1.5*10^8$ but would still be the same speed, right?  
> **username_6**: @blankettripod Yes. But in order to match our current models, either $c$ would have to change, or we'd have to speed up / slow down as we changed size. (Positing that we're growing / shrinking, _and_ all the physical constants are changing in exactly the way that'd prevent us from noticing this, is a more complicated theory than just assuming we're not.)  
> **username_2**: I have heard that light speed c is nowadays defined as a fixed value and meter is now defined as variable - as whatever distance that light passes during one second. Not the way around. Is that true? If so, then the length of a meter would change without c value changing. Do I get that right?  
> **username_1**: @RolandPihlakas That new physics-based definition of the International System of Units (SI for short; the acronym is French) assumes the speed of light is constant. There is absolutely no reason to disbelieve this. Given ever improving observations that the speed of light is indeed constant (and that various other physical quantities are constant), the SI community decided about four years ago to use those constants of nature as the basis for a complete revamping of the SI base units.  
> **username_2**: @David Hammen The objective of the question is not whether I am disbelieving that speed of light is constant. Instead I am asking what are the consequences of this sort of setup. Is a meter potentially changing its length? Just as with your other response and comment, again I am surprised that you similar theme here "There is absolutely no reason to disbelieve this" - focusing on forcing my beliefs instead of responding to the content of the question. Why?  
> **username_7**: Interestingly enough, I once did a calculation on how much energy would be radiated if all matter in the universe were shrinking, based on the idea that mass would decrease concordantly.  It came out to a pretty reasonable value (small enough that we probably wouldn't detect it locally) as well as comparable in total to the estimated dark energy content of the universe.  

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### Answer by username_1 on 2023-02-01. Score: 15

We do not know whether or not we live in a simulation in which our capricious simulation overlords have conspired to hide evidence that we are growing larger. Discounting that possibility, science says we are not growing larger.

The expansion of the universe is something that happens at very large distance scales. At moderate distance scales gravitation overwhelms expansion, at least for now, and will continue to do so for trillions of years in the future. At even smaller scales (e.g., us), electromagnetic interactions overwhelm gravity.

We are not expanding, nor is the Earth, nor is the solar system, nor is the galaxy. Our galaxy is on a headlong course to collide with the Andromeda Galaxy in about five billion years due to gravitation. The expansion of the universe is something that happens on an even larger scale.

> **Comments:**
> **username_2**: This does not answer the question: how do we know? It just tells what we know, as if "take it or leave it".  
> **username_1**: @RolandPihlakas It does answer the question as far as I'm concerned. One either accepts that science is more or less correct, which clearly says we're not getting bigger for reasons elaborated in the answer, or one goes to the woo side of pseudoscience that says anything goes. That woo side is impossible to disprove. What if there's some malevolent super-being who has made key observations we make somewhat fictional? What if we're living in a simulation made by capricious super-beings who decided to toy with the simulated minds of their simulated under-beings (e.g., us)?  
> **username_2**: @ David Hammen The question *explicitly* asked "how" do we know, not "what" we know. I other words, OP wanted to understand. This response does not even attempt to answer the "how" question. It could be even argued that believing the "science" without understanding it is closer to pseudoscience than the desire to understand would be. How is one going to learn science if the responses are like "just trust in the science"? I am surprised that you dismissed my concern.  
> **username_1**: @RolandPihlakas The answer is indeed just trust in the science. As my edit says, we do not and cannot know whether we are simulated beings. I prefer to think we are not. Accepted science says the expansion is a very large scale phenomenon while at small scales (groups of galaxies are "small scale"), the interactions we know of completely overwhelm the expansion of space.  

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### Answer by username_8 on 2023-02-02. Score: 10

Beyond the other answers here, we *can* measure an upper limit on how fast we're growing. For example, imagine that everything doubled in size. You're right that our rulers would double in size and everything would seems the same size from that perspective.

However, since everything is twice as big, light now needs twice as long to travel the distances that it used to. Every network engineer using fibreoptic cables suddenly sees their latency double. Every GPS unit fails, as the calculation is partially dependent on using the time it takes the signal to travel from the satellite to the device. Radios would fail as the frequency controlled by the electronics would no longer match the wavelength optimized by the antenna (though this wouldn't be quite as prfound if it was truly an exact doubling).

To be more succinct, the speed of light is *defined* as 299,792,458 meters per second. The length of the meter and the duration of the second are linked by this fundamental definition. If something comes along and changes the meter, then the second is redefined to match. If the duration of a second was changing, it would be pretty noticeable.

> **Comments:**
> **username_9**: Perhaps light gets faster? Perhaps we will feel the growth, it depends how linear it is  
> **username_10**: "If the duration of a second was changing, it would be pretty noticeable." - why wouldn't our perception of it change to match the actual change?  

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### Answer by username_11 on 2023-02-02. Score: 6

**Space is not expanding,** at least not in the way that you are thinking. Expanding space is just a convention that simplifies some of the mathematics in the context of cosmology. The contents of the universe are expanding -- moving apart in a uniform way -- and this process is conveniently described using a coordinate system in which space is taken to expand. But that is just a convention. Expanding space does not have local physical consequences.

This means that the expansion of space does not cause objects to expand. Not only that, *it doesn't even supply an expansion force that has to be counteracted by gravity or electromagnetism*, as is often mistakenly believed.

**Why then is the universe expanding, if not because of the expansion of space?** Imagine throwing a ball up into the air at escape velocity. After the ball departs, there is no continued impetus for separation; indeed gravity is pulling the ball back. And yet the ball continues to move away from the Earth due its initial motion alone. It's similar with the universe. Cosmic expansion is a consequence of motion in the initial conditions.

### Notes

There is one physical sense in which space might be expanding. If the universe is finite, then its total volume (measured on comoving surfaces) grows. This does not affect local dynamics, though.

Also, dark energy supplies an expansion force. However, this is more clearly viewed as *the cause of* accelerated expansion than a consequence of it. Also, the force itself is most clearly interpreted as just gravity, since it arises from the equations of general relativity.

### Further reading

I also include some choice quotes.

* [A diatribe on expanding space.](https://arxiv.org/abs/0809.4573) "This analysis demonstrates that there is no local effect on particle dynamics from the global expansion of the universe: the tendency to separate is a kinematic initial condition, and once this is removed, all memory of the expansion is lost."
* [The kinematic origin of the cosmological redshift.](https://arxiv.org/abs/0808.1081) "The tendency [of the Solar System] to expand due to the stretching of space is nonexistent, not merely negligible."
* [On The Relativity of Redshifts: Does Space Really "Expand"?](https://arxiv.org/abs/1605.08634) "But if you assume that expanding space is something physical, something like a river carrying distant observers along as the universe expands,
  the consequence of this when considering the motions of objects in the universe will lead to radically incorrect results."

> **Comments:**
> **username_2**: Since when does the space not expand? Was big bang expansion of space or the particles moving away from each other? I have heard so far that big bang was expansion of space, so did something change later?  
> **username_11**: @RolandPihlakas As [Steven Weinberg said](https://www.newscientist.com/article/mg13818693-600/), "*Cosmologists sometimes talk about expanding space – but they should know better.*" To be clear, cosmology is a mathematical science, and everyone agrees on the mathematics. There is no question about the mathematical, and hence physical, description of an expanding universe. The only question is how we translate the mathematics into words or pictures. And "expanding space" is a translation that leads to a lot of misconceptions, like the one that led to this question.  
> **username_12**: Dear @Sten, The statement that "The tendency of the Solar System to expand due to the stretching of space is nonexistent" is incorrect. https://www.sciencedaily.com/releases/2012/10/121009161103.htm  
> **username_11**: @Michael_1812 That paper's conclusion would imply that cosmic expansion affects local dynamics in the [Milne universe](https://en.wikipedia.org/wiki/Milne_model), which is impossible because the Milne universe is just re-parametrized Minkowski spacetime. The paper is wrong.  
> **username_12**: @Sten The linear FLRW is mathematically equiv to Milne, which is mathematically equiv to a global Minkowski spacetime. However,  the global Minkowski time T is not the local Minkowski time, and is not directly observable. We measure the local Minkowski time, (the proper time tau). In Milne, photons have a constant velocity in the global Minkowski spacetime. So, the tau measured by our clocks and the T along light geodesics are not the same. Kopeikin proves that the difference is measurable if our local space is expanding. His result reproduces the Pioneer anomaly, both in magnitude and sign  
> **username_11**: @Michael_1812 Are you suggesting that changing the way you parametrize spacetime changes the physical behavior of clocks?  
> **username_12**: @Sten On the contrary, I emphasise that there is a difference between the proper time \tau and the coordinate time T. This difference is exploited in Kopeikin's work: his paper is about kinematics of clocks (timelike geodesics) and light particles (null geodesics) in two different metrics - one is globally flat and another one has a conformal factor depending on the time tau in front of the spatial metric. The Milne coordinates and the local coordinates are not identical - they are connected by a non-linear transformation.  
> **username_11**: @Michael_1812 Milne coordinates don't matter. They are just coordinates. If the article by S. Kopeikin is correct, then the influence of cosmic expansion is expected *in special relativity*, and that influence could be of any imaginable magnitude, since Minkowski spacetime can be parametrized with Milne coordinates with any expansion rate.  
> **username_12**: @Sten We measure the proper time \tau. Also, please be mindful that global expansion exists in all viable cosmological models, which models look the same in the linear order of approximation with respect to the Hubble constant. Within this approximation, it is impossible to distinguish (mathematically) the Milne model from the FLRW one. To do so, one would have to extend his calculations to the second Hubble order, which Kopeikin did not do, because H^2 terms are too small to be measurable in the Pioneer experiment.  
> **username_13**: This answer's cited creates a distinction between vacuum energy and the hubble constant/space expansion, who then say "vacuum energy makes things move apart, but that isn't space expansion".   That is rather ahistorical; vacuum energy was created as a mechanism to explain space expansion.  Saying "the thing that explains X means X does not exist" seems, well, fundamentally wrong.  
> **username_13**: "In the case of vacuum-dominated models, then, the repulsive effects of vacuum energy cause all pairs of particles to separate at large times, whatever their initial kinematics; this behaviour could perhaps legitimately be called ‘expanding space’. Nevertheless, the effect stems from the clear physical cause of vacuum repulsion, and there is no new physical influence that arises purely from the fact that the universe expands. The earlier examples have proved that ‘expanding space’ is in general a dangerously flawed way of thinking about an expanding universe." arXiv:0809.4573 [astro-ph]  
> **username_11**: @Yakk-AdamNevraumont The distinction comes from the mathematics. It's possible to interpret dark energy's repulsion as a consequence of "stretching of space", but that interpretation is (1) only valid for one particular coordinate system and (2) only valid if dark energy is truly a cosmological constant (we don't know that). [General covariance](https://en.wikipedia.org/wiki/General_covariance) is a central principle of general relativity, and the "stretching of space" interpretation of the action of dark energy does not admit a covariant description.  
> **username_11**: @Yakk-AdamNevraumont Also, dark energy was *not* created to explain cosmic expansion! Cosmic expansion would happen with or without dark energy. Dark energy was only devised to explain a particular feature in the expansion history.  
> **username_14**: @Sten how do you explain a redshift of z=5?  
> **username_11**: @LawnmowerMan What's special about $z=5$? Is this a concern about things receding "faster than light"? The answer to such concerns is that those recession rates are not relative velocities. If you were to define the recession rate of your friend in a rocket traveling away from you at near light speed in the same way that we define the recession rate of a cosmologically distant object, you would also conclude that your friend is receding much faster than light.  
> **username_11**: In fact, there's a special case of the expanding universe solution (the [FLRW metric](https://en.wikipedia.org/wiki/FLRW)) called the [Milne model](https://en.wikipedia.org/wiki/Milne_model) (mentioned in earlier comments). It's special because it turns out to be exactly equivalent to (a portion of) the flat, static spacetime of special relativity. In spite of this, the model includes features people normally attribute to expanding space, like faster-than-light recession rates. The Milne model is one of the clearest demonstrations that "expanding space" is just a coordinate choice.  
> **username_14**: @Sten it also requires zero energy density and negative curvature, both of which are incompatible with cosmological observations. Doesn't this put you in the crank corner of physics?  
> **username_11**: @LawnmowerMan What is this strawman? It's an illustrative example, showing how you can faster than light recession with no concept of expanding space.  
> **username_14**: @Sten ok, then please explain how the observable universe is > 20 gly across but only 13 gy old. You have to reject BB/inflation or size estimates or you have FTL galaxies. Which do you pick?  
> **username_11**: @LawnmowerMan It's essentially time dilation. The key issue is that there is no objective definition of "now" for observers in different places. In cosmology, we choose to define "now" for a distant observer to mean "the same amount of time has elapsed since the Big Bang". But observers receding at relativistic speeds are time dilated. If an observer is receding from us at a relativistic speed, they could cover a distance much greater than 13 billion light years in an "as measured by them" (proper) time span of only 13 billion years.  
> **username_11**: That's not an exact picture, since (for realistic cosmologies, i.e. not the Milne model) spacetime is globally curved. In such spacetimes, you can't even uniquely define a relative velocity (and hence time dilation factor) for a distant object, so it's more difficult to give an intuitive account for what's happening. Nevertheless, it should remain qualitatively clear that relativistic considerations can explain the observable universe being larger than the age of the universe.  
> **username_14**: @Sten I think your argument is disingenuous. ΛCDM is the best-supported model and does explicitly include expansion of space. I think the strongest statement you can make is that a Milne-style cosmology might be compatible with observations, if only you can find all the missing antimatter. But pretending that it is the "true description" of reality while ΛCDM is a poorly described alternative is not the least bit justifiable.  
> **username_11**: @LawnmowerMan Regarding "ΛCDM is the best-supported model and does explicitly include expansion of space", the whole point is that this is false. It doesn't matter if you are considering ΛCDM, Milne, de Sitter, Einstein-de Sitter, whatever. The basic fact is that there is no such thing as an objective notion of "expansion of space" within general relativity. Spacetime simply doesn't include properties like dilation and motion. The only objective physical property of a point in spacetime is its (tensor) curvature.  
> **username_11**: @LawnmowerMan The only reason I brought in Milne is that it is easier to develop an intuition for. It's not special otherwise.  

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### Answer by username_15 on 2023-02-02. Score: 6

If the expansion affected things at the scale of stars and galaxies, we would see galaxies get smaller the further away they are. Since that is not the case, we can deduce that the expansion doesn't happen at those scales.

> **Comments:**
> **username_2**: I think you meant to say, if the expansion affected galaxies then they would get bigger...  
> **username_16**: well another way to say that is that if everything was getting bigger at a constant rate, things that were closer to us would _appear_ to get bigger faster because apparent size has an inverse relationship to distance, which ends up being the same thing  
> **username_15**: @RolandPihlakas they would get smaller the further away they were.  Due to the light travel time, we would be seeing them at a time when they had expanded less.  
> **username_17**: Distant galaxies *are* smaller on average (but not for this reason).  

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### Answer by username_12 on 2023-02-04. Score: 1

In a certain sense, we *are* getting bigger -- where "we" means: the solar system. I am talking about a physical effect, which is so feeble that it took astronomers dozens of years to measure it reliably. This is the so-called [Pioneer anomaly](https://en.wikipedia.org/wiki/Pioneer_anomaly).

It turned out that the two Pioneer spacecraft launched in the early 70s have been decelerating anomalously as they were departing the solar system.

Although the topic is still being discussed, the cosmological expansion seems to be the likeliest reason for this anomaly. See, e.g., this [press release](https://www.sciencedaily.com/releases/2012/10/121009161103.htm) and a reference therein.

> **Comments:**
> **username_18**: The consensus is that the Pioneer anomaly was caused by thermal emissions. The paper by Kopeikin mentioned in that press release (available non-paywalled [here](https://www.researchgate.net/publication/233967292_Celestial_dynamics_and_astrometry_in_expanding_universe)) is completely wrong and it's pretty shocking to me that it was accepted by Physical Review D. It's the same error as in the paper of Cooperstock et al. mentioned in [this answer](//physics.stackexchange.com/a/601323): he's calculating the gravitational effect of nonexistent matter.  
> **username_12**: @benrg The work by Kopeikin was accepted by Phys Rev D for two reasons. First, it underwent *very* thorough refereeing. Second, Kopeikin is one of the world leading experts on GR. Last, and by no means least, there is no established consensus on the Pioneer anomaly.  
> **username_19**: @Michael_1812 that's interesting; one can calculate the thrust due to recoil from infrared photons radiated from one side of the spacecraft (the hot side of course) and one can know that this must happen. **In other words, *it would be a huge puzzle if this deviation was not seen!*** Related in Space SE: [Did New Horizons also demonstrate the "Pioneer Anomaly"?](https://space.stackexchange.com/q/15075/12102) (with images from simulation) and [Could radiated heat propel space-craft in outer space?](https://space.stackexchange.com/q/3591/12102)  
> **username_19**: @Michael_1812 and [Is the speed of the Voyager probes accurately described by Newton's or Einstein's theories?](https://space.stackexchange.com/q/14146/12102) (yes) So for "there is no established consensus" to be true, I think that you have to include flat-Earthers and conspiracy theorists in your pool of opinions.  

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### Answer by username_20 on 2023-02-02. Score: 1

If everything is getting bigger at scale, so that there is no possible way to measure it, than we can't measure it. But if there is no way to measure it, there is no influence on anything. And if it has no influence, it doesn't matter, if it does.


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### Answer by username_21 on 2023-02-18. Score: 1

In a closed universe, after expanding, the universe contracts. Locally, we are in a closed region called the Local Group, and the Andromeda galaxy is approaching. Now are we expanding to explain the collapse of the Local Group, or are we shrinking to explain large scale expansion? (We are close to stationary with respect to the Virgo Supercluster, just to make it more problematic). Clearly, neither shrinking nor expanding would work to explain all of this.

GR just says that space is subjected to curvature from mass-energy, and free falling motion is always along a geodesic. If you start with particles all moving away from each other, they may continue to do so for a while, but if they have mass then the curvature of space-time slows them down. If the local density is high enough then this material is destined to fall together. But, if there are centrifugal forces or electrical forces, as in the solid Earth, to oppose the fall, then the motion can be stopped.


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