<p>Once we look far enough away to get past "local effects" we see everything moving apart, spreading out in all directions, like a set of dots on a balloon being inflated, or like currants in a loaf of bread rising in the oven.</p>

<p>We can use the laws of physics to follow those movements back in time to work out how far apart all the galaxies were at various dates in the past. If we do this we find that 13.8 billion years ago they were all in the same place (there are a lot of subtle details to get right in this calculation, but a lot of people have worked on it over a long time). That much matter and energy packed into that small a space warps both space and time so that it is not really possible to talk about time "before" that event. Hence that is taken as the origin of the universe.</p>

CC BY-SA 4.0
2020-04-29T22:15:44.060Z
But how can we be sure that the rate at which two points are moving apart has always been same constant throughout billions of years? Certainly we only can experience the tiny fraction of that huge time right?
Sazzad Hissain Khan
CC BY-SA 4.0
2020-04-29T22:30:58.480Z
@SazzadHissainKhan Basically, things in space keep moving at the same speed in the same direction unless there is a force acting on them. On these scales, the only force that matters in gravity and we can calculate that and allow for it. Less basically, the matter in space, its movement and the shape of space itself are all interrelated, according to General Relativity, but we can use the equations of GR to track all of these things back in time and allow for the forces.
Steve Linton
CC BY-SA 4.0
2020-05-01T07:36:18.727Z
@SteveLinton This seems like a simplification; dark energy is known to have a large effect, but we cannot explain it yet.
TheGreatCabbage
CC BY-SA 4.0
2020-05-01T07:57:03.463Z
@TheGreatCabbage  We can’t explain it, but we can measure the effect and allow for it
Steve Linton
CC BY-SA 4.0
2020-05-03T15:00:41.600Z
@SteveLinton as you said "unless there is a force acting on them". My question was, are we sure about the claim there was no unidentified/undiscovered force throughout billions of years? Did science already get the evidence or not? If yes, how?
Sazzad Hissain Khan
CC BY-SA 4.0
2024-12-23T16:43:34.163Z
The further away something is, the further back in time we see it.  So we can see the rate of expansion at many points in the past and verify it is the same.
Greg Miller
CC BY-SA 4.0
2020-04-29T22:07:32.963Z
2020-04-30T08:26:50.357Z
2020-04-30T08:26:50.357Z
Steve Linton
<p>In the same way that if you were to measure the height and speed of something thrown into the air, you would be able to work out how hard it had been thrown and how long ago. The assumption is made that the trajectory of the thrown object obeys the known laws of physics.</p>
<p>A further advantage one has in astronomy is that you are not limited to making a single measurement in the present day, you can observe distant things as they were in the past. This gives you multiple opportunities to measure the size and expansion rate of the universe to corroborate your answer and/or uncover holes in  understanding of the physics (like dark energy for example). Nevertheless, to some extent, the age you quoted does assume we have the physics right.</p>
<p>At the moment, the age estimate is heavily dependent on measurements of features that formed when the universe was about 400,000 years old (the cosmic microwave background), combined with observations of supernovae in galaxies at a range of distances (and times in the past), that tell us precisely how the trajectory of the universe has changed with time.</p>
<p>The accuracy of the answer (as opposed to the <em>precision</em>, which is very high), can be verified with independent tests like estimating the ages of the oldest stars (which appear to be around 12 billion years old; suitably younger than the actual universe!), looking at how much structure there is in the universe, both now and in the past, using very long-lived isotope dating (e.g. the universe should be older than the Earth!) etc. This is a useful exercise because these methods use partly independent physics to arrive at ages/timescales.</p>
<p>Basically, everything ties up <em>reasonably</em> well at the moment (in fact there are small discrepancies in the expansion rate measured now, compared with predictions from the ancient cosmic microwave background), so the age you quote is an age for the &quot;concordance model&quot;, that uses an agreed (though not entirely understood) set of physics.</p>
<p>Also probably worth stating that although this age is often referred to as &quot;the age of the universe&quot;, it may not be. It is the length of time that has passed since the universe was much, much smaller than it is now. In this first fraction of a second, we reach the limits of the physics at these high densities and temperatures. For example, it is entirely possible that the universe existed for some time before this, or perhaps even had an oscillatory nature and therefore could be more ancient. But this is in the realm of speculation.</p>

CC BY-SA 4.0
2020-04-30T07:41:27.430Z
2024-12-23T13:39:52.440Z
2024-12-23T13:39:52.440Z
ProfRob
<p>As mentioned in <a href="https://en.wikipedia.org/wiki/Age_of_the_universe" rel="noreferrer">wiki/Age_of_the_universe</a>,</p>

<blockquote>
  <p>The current measurement of the age of the universe is around 13.8 billion years (as of 2015) – 13.799±0.021 billion years</p>
</blockquote>

<p>When my friend who is not from science background asks me <strong>how do we measure the age of the universe?</strong> How should I explain to him?</p>

<universe><time><age><observational-astronomy>
How do we measure the age of the universe?
Sazzad Hissain Khan
# How do we measure the age of the universe?

(Asked by: username_0 on 2020-04-29)

As mentioned in [wiki/Age\_of\_the\_universe](https://en.wikipedia.org/wiki/Age_of_the_universe),

> The current measurement of the age of the universe is around 13.8 billion years (as of 2015) – 13.799±0.021 billion years

When my friend who is not from science background asks me **how do we measure the age of the universe?** How should I explain to him?

**Tags:** universe, time, age, observational-astronomy

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## 2 answers

### Answer by username_4 on 2020-04-30. Score: 6 | Accepted answer

In the same way that if you were to measure the height and speed of something thrown into the air, you would be able to work out how hard it had been thrown and how long ago. The assumption is made that the trajectory of the thrown object obeys the known laws of physics.

A further advantage one has in astronomy is that you are not limited to making a single measurement in the present day, you can observe distant things as they were in the past. This gives you multiple opportunities to measure the size and expansion rate of the universe to corroborate your answer and/or uncover holes in understanding of the physics (like dark energy for example). Nevertheless, to some extent, the age you quoted does assume we have the physics right.

At the moment, the age estimate is heavily dependent on measurements of features that formed when the universe was about 400,000 years old (the cosmic microwave background), combined with observations of supernovae in galaxies at a range of distances (and times in the past), that tell us precisely how the trajectory of the universe has changed with time.

The accuracy of the answer (as opposed to the *precision*, which is very high), can be verified with independent tests like estimating the ages of the oldest stars (which appear to be around 12 billion years old; suitably younger than the actual universe!), looking at how much structure there is in the universe, both now and in the past, using very long-lived isotope dating (e.g. the universe should be older than the Earth!) etc. This is a useful exercise because these methods use partly independent physics to arrive at ages/timescales.

Basically, everything ties up *reasonably* well at the moment (in fact there are small discrepancies in the expansion rate measured now, compared with predictions from the ancient cosmic microwave background), so the age you quote is an age for the "concordance model", that uses an agreed (though not entirely understood) set of physics.

Also probably worth stating that although this age is often referred to as "the age of the universe", it may not be. It is the length of time that has passed since the universe was much, much smaller than it is now. In this first fraction of a second, we reach the limits of the physics at these high densities and temperatures. For example, it is entirely possible that the universe existed for some time before this, or perhaps even had an oscillatory nature and therefore could be more ancient. But this is in the realm of speculation.


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### Answer by username_1 on 2020-04-29. Score: 4

Once we look far enough away to get past "local effects" we see everything moving apart, spreading out in all directions, like a set of dots on a balloon being inflated, or like currants in a loaf of bread rising in the oven.

We can use the laws of physics to follow those movements back in time to work out how far apart all the galaxies were at various dates in the past. If we do this we find that 13.8 billion years ago they were all in the same place (there are a lot of subtle details to get right in this calculation, but a lot of people have worked on it over a long time). That much matter and energy packed into that small a space warps both space and time so that it is not really possible to talk about time "before" that event. Hence that is taken as the origin of the universe.

> **Comments:**
> **username_0**: But how can we be sure that the rate at which two points are moving apart has always been same constant throughout billions of years? Certainly we only can experience the tiny fraction of that huge time right?  
> **username_1**: @SazzadHissainKhan Basically, things in space keep moving at the same speed in the same direction unless there is a force acting on them. On these scales, the only force that matters in gravity and we can calculate that and allow for it. Less basically, the matter in space, its movement and the shape of space itself are all interrelated, according to General Relativity, but we can use the equations of GR to track all of these things back in time and allow for the forces.  
> **username_2**: @SteveLinton This seems like a simplification; dark energy is known to have a large effect, but we cannot explain it yet.  
> **username_1**: @TheGreatCabbage  We can’t explain it, but we can measure the effect and allow for it  
> **username_0**: @SteveLinton as you said "unless there is a force acting on them". My question was, are we sure about the claim there was no unidentified/undiscovered force throughout billions of years? Did science already get the evidence or not? If yes, how?  
> **username_3**: The further away something is, the further back in time we see it.  So we can see the rate of expansion at many points in the past and verify it is the same.  

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