<p>I would like to point out that the ashphalt isn't the only reason that it wouldn't expand. Earth's gravity would keep it together regardless. But say we ignore that.</p>
<p>I would also like to point that the speed at which the universe expands depends on how far away it is. Since you asked how far a mile stretches, let's say you are at the beginning of the mile, and we'll figure out how far the end of the mile is after 80 years.</p>
<p>Final nitpicky thing, but a mile is a set distance and doesn't expand. I'll assume you mean the distance between 2 objects which started a mile apart.</p>
<p><a href="https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095948562#:%7E:text=This%20gives%20the%20distance%20from,radius%20of%2013.7%20billion%20l.y" rel="nofollow noreferrer">The expansion speed of the universe is 71 km/s/MPC.</a> MPC stands for Mega-Parsec. A Mega-Parsec is about 19.2 quintillion miles. So the expansion speed a mile away from you is about 0.0000000000000000036979167 km/s. That's about 0.0000000000000133125 km/h, or 0.00000000011669738 km/year. Over 80 years, the mile will expand by only 0.00000000933579001 kilometers, or 0.00000933579001 meters. If you want less zeros, maybe calling it 9.33679001 micrometers is the way to go. If you want imperial units, then 0.0003675507877953 inches it is.</p>
<p>This means you're estimate of 3 microns was likely incorrect unless I did something very wrong here (please note that micrometers and microns are the same thing).</p>
<p>Please note that I am ignoring the incredibly small effect that the very slight expansion of the mile will have on increasing its future expansion rate. Considering that the total expansion is only 0.00000000580099097 of a mile, it would have virtually no effect.</p>

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2024-11-12T22:52:19.000Z
2024-11-13T01:19:28.017Z
2024-11-13T01:19:28.017Z
Starship
<p>Cosmic expansion is not a force or dynamical influence. It's just a description of what the contents of the universe are doing. At scales much larger than galaxies, those contents are moving apart, such that the average density of the universe is dropping.</p>
<p>Expansion of <em>space itself</em> is not a physical phenomenon; it's just a mathematical convention (<a href="https://astronomy.stackexchange.com/a/53836/47607">see my detailed discussion here</a>). Cosmic expansion does not stretch things any more than the diverging meridian lines would stretch your body as you walk north from the South Pole.</p>

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2024-11-13T00:55:23.733Z
2024-11-14T16:34:46.397Z
2024-11-14T16:34:46.397Z
Sten
<p>A mile doesn't stretch as it is a defined distance unsusceptible to expansion.</p>
<p>The currently accepted rate for the cosmic expansion as given by the Hubble constant is <span class="math-container">$H_0 \approx 70$</span> km/s/Mpc where <span class="math-container">$1 Mpc \approx 3\cdot 10^{19}$</span>km. For simplicity's sake let's assume you will live <span class="math-container">$t_{life} = 100$</span> years or <span class="math-container">$365\cdot 100\cdot 3600\cdot 24\approx 3\cdot 10^9$</span> seconds.</p>
<p>So space expands in your lifetime by about <span class="math-container">$d_l = H_0\cdot t_{life} = 70\cdot 10^{-10} \mathrm{km/km}\approx 7\cdot 10^{-9}\mathrm{km/km} \approx 7\mu\mathrm{m}/km$</span> or about 10µm / mile.</p>
<p>However no physical body (planet, star, not even solar system or galaxy) is being stretched as the forces which keeps things together dominate over cosmic expansion by many orders of magnitude.</p>

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2024-11-13T01:20:35.190Z
The Hubble constant is 71 km/s/Mpc not 70, see https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095948562#:~:text=This%20gives%20the%20distance%20from,radius%20of%2013.7%20billion%20l.y. And, other than having the wrong value for the Hubble constant, this answer is no different from mine. This also doesn't answer the question asked, MU specified 80 years.
Starship
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2024-11-14T00:29:41.100Z
@Starship the Hubble parameter is not yet known to an accuracy of 2 significant figures.
ProfRob
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2024-11-14T00:31:23.257Z
@ProfRob Then why do plenty of sources claim to know it?
Starship
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2024-11-14T00:34:05.510Z
@Starship there is currently great debate about the value of the Hubble parameter. Values from about 67 to 73 km/s/Mpc are quoted. Nobody knows what the value is to 2 significant figures, b
ProfRob
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2024-11-14T00:39:35.947Z
@ProfRob My point is why do people claim to know the value then? If you don't know something, why pick "a random value it could be" rather than saying you don't know?
Starship
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2024-11-14T00:49:22.417Z
@Starship "people" don't. The current uncertainties are about +/-2 km/s/Mpc at the 1 sigma level.
ProfRob
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2024-11-12T23:38:06.030Z
2024-11-12T23:38:06.030Z
planetmaker
<p>I've wanted to figure this out since I was a kid. Cosmic expansion is very small locally, and I was trying to get an appreciation for it.</p>
<p>How much would the mile expand in 80 years if the electrons in the asphalt weren't continually pulling the asphalt nuclei back together?</p>
<p>Every second, the space between us and galaxies 3.2 million light years away expands by 71 km.  That sounds like a lot, but it's a lot of distance.</p>
<p>The ratio is 2 quintillionths. Multiply any distance by that, and you find out how much the distance expands every second.</p>
<p>A year is 32 million seconds. But every time I try to figure this out, I get a different answer — most of them wildly ridiculous — because I literally can't do 3rd grade arithmetic (I can do it, but I get confused about which numbers mean what).</p>
<p>What's the correct answer? My best estimate is 3 microns, but I'm not very confident of it.</p>

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2024-11-12T22:35:38.277Z
Just curious, how did you get an estimate of 3 microns.
Starship
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2024-11-12T23:39:46.457Z
@Starship 
Apparently the right way, because I'm only off by a factor of 3. Last time I tried, I got several feet/year. But I did the arithmetic by asking Alexa and keeping track of everything in my head, so that might explain it.
Miss Understands
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2024-11-13T00:11:24.413Z
https://en.wikipedia.org/wiki/Hubble%27s_law#21st_century_measurements lists a variety of recent estimates for the expansion rate, from 65 to 75 km/s/Mpc, so 71 is a reasonable value.
PM 2Ring
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2024-11-13T01:17:50.713Z
@Miss_Understands What I was asking is what math where you doing?
Starship
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2024-11-13T03:53:21.553Z
@PM2Ring I used 71 because it's the value obtained when you compute H in a simple, [completely different way](https://astronomy.stackexchange.com/questions/58837/you-can-easily-derive-the-hubble-constant-from-the-age-of-the-universe-and-c-is) that never occurred to anyone before. It uses only the age and assumes constant expansion at c.
Miss Understands
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2024-11-13T05:51:05.523Z
@Miss_Understands    A year is not 32 million seconds.  The number will vary by which type of Earth year you use, but it will be over 31 million seconds and less than 32 million.  There are 86,400 seconds in a day, and the average Gregorian Calendar year is 365.2425 days long, thus making 31,556,952 seconds in an average Gregorian calendar year.
M. A. Golding
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2024-11-13T05:55:53.597Z
(jee-ziss christ) Alexa said "A year one year is 31.54 million seconds." That's where I got 32.
Miss Understands
<distances><expansion><hubble-constant>
How much does a mile stretch in my lifetime due to cosmic expansion?
Miss Understands
# How much does a mile stretch in my lifetime due to cosmic expansion?

(Asked by: username_0 on 2024-11-12)

I've wanted to figure this out since I was a kid. Cosmic expansion is very small locally, and I was trying to get an appreciation for it.

How much would the mile expand in 80 years if the electrons in the asphalt weren't continually pulling the asphalt nuclei back together?

Every second, the space between us and galaxies 3.2 million light years away expands by 71 km. That sounds like a lot, but it's a lot of distance.

The ratio is 2 quintillionths. Multiply any distance by that, and you find out how much the distance expands every second.

A year is 32 million seconds. But every time I try to figure this out, I get a different answer — most of them wildly ridiculous — because I literally can't do 3rd grade arithmetic (I can do it, but I get confused about which numbers mean what).

What's the correct answer? My best estimate is 3 microns, but I'm not very confident of it.

**Tags:** distances, expansion, hubble-constant
> **Comments:**
> **username_1**: Just curious, how did you get an estimate of 3 microns.  
> **username_0**: @Starship  Apparently the right way, because I'm only off by a factor of 3. Last time I tried, I got several feet/year. But I did the arithmetic by asking Alexa and keeping track of everything in my head, so that might explain it.  
> **username_2**: https://en.wikipedia.org/wiki/Hubble%27s_law#21st_century_measurements lists a variety of recent estimates for the expansion rate, from 65 to 75 km/s/Mpc, so 71 is a reasonable value.  
> **username_1**: @Miss_Understands What I was asking is what math where you doing?  
> **username_0**: @PM2Ring I used 71 because it's the value obtained when you compute H in a simple, [completely different way](https://astronomy.stackexchange.com/questions/58837/you-can-easily-derive-the-hubble-constant-from-the-age-of-the-universe-and-c-is) that never occurred to anyone before. It uses only the age and assumes constant expansion at c.  
> **username_3**: @Miss_Understands    A year is not 32 million seconds.  The number will vary by which type of Earth year you use, but it will be over 31 million seconds and less than 32 million.  There are 86,400 seconds in a day, and the average Gregorian Calendar year is 365.2425 days long, thus making 31,556,952 seconds in an average Gregorian calendar year.  
> **username_0**: (jee-ziss christ) Alexa said "A year one year is 31.54 million seconds." That's where I got 32.  

---

## 3 answers

### Answer by username_1 on 2024-11-12. Score: 2 | Accepted answer

I would like to point out that the ashphalt isn't the only reason that it wouldn't expand. Earth's gravity would keep it together regardless. But say we ignore that.

I would also like to point that the speed at which the universe expands depends on how far away it is. Since you asked how far a mile stretches, let's say you are at the beginning of the mile, and we'll figure out how far the end of the mile is after 80 years.

Final nitpicky thing, but a mile is a set distance and doesn't expand. I'll assume you mean the distance between 2 objects which started a mile apart.

[The expansion speed of the universe is 71 km/s/MPC.](https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095948562#:%7E:text=This%20gives%20the%20distance%20from,radius%20of%2013.7%20billion%20l.y) MPC stands for Mega-Parsec. A Mega-Parsec is about 19.2 quintillion miles. So the expansion speed a mile away from you is about 0.0000000000000000036979167 km/s. That's about 0.0000000000000133125 km/h, or 0.00000000011669738 km/year. Over 80 years, the mile will expand by only 0.00000000933579001 kilometers, or 0.00000933579001 meters. If you want less zeros, maybe calling it 9.33679001 micrometers is the way to go. If you want imperial units, then 0.0003675507877953 inches it is.

This means you're estimate of 3 microns was likely incorrect unless I did something very wrong here (please note that micrometers and microns are the same thing).

Please note that I am ignoring the incredibly small effect that the very slight expansion of the mile will have on increasing its future expansion rate. Considering that the total expansion is only 0.00000000580099097 of a mile, it would have virtually no effect.


---

### Answer by username_4 on 2024-11-13. Score: 2

Cosmic expansion is not a force or dynamical influence. It's just a description of what the contents of the universe are doing. At scales much larger than galaxies, those contents are moving apart, such that the average density of the universe is dropping.

Expansion of *space itself* is not a physical phenomenon; it's just a mathematical convention ([see my detailed discussion here](https://astronomy.stackexchange.com/a/53836/47607)). Cosmic expansion does not stretch things any more than the diverging meridian lines would stretch your body as you walk north from the South Pole.


---

### Answer by username_5 on 2024-11-12. Score: 1

A mile doesn't stretch as it is a defined distance unsusceptible to expansion.

The currently accepted rate for the cosmic expansion as given by the Hubble constant is $H\_0 \approx 70$ km/s/Mpc where $1 Mpc \approx 3\cdot 10^{19}$km. For simplicity's sake let's assume you will live $t\_{life} = 100$ years or $365\cdot 100\cdot 3600\cdot 24\approx 3\cdot 10^9$ seconds.

So space expands in your lifetime by about $d\_l = H\_0\cdot t\_{life} = 70\cdot 10^{-10} \mathrm{km/km}\approx 7\cdot 10^{-9}\mathrm{km/km} \approx 7\mu\mathrm{m}/km$ or about 10µm / mile.

However no physical body (planet, star, not even solar system or galaxy) is being stretched as the forces which keeps things together dominate over cosmic expansion by many orders of magnitude.

> **Comments:**
> **username_1**: The Hubble constant is 71 km/s/Mpc not 70, see https://www.oxfordreference.com/display/10.1093/oi/authority.20110803095948562#:~:text=This%20gives%20the%20distance%20from,radius%20of%2013.7%20billion%20l.y. And, other than having the wrong value for the Hubble constant, this answer is no different from mine. This also doesn't answer the question asked, MU specified 80 years.  
> **username_6**: @Starship the Hubble parameter is not yet known to an accuracy of 2 significant figures.  
> **username_1**: @ProfRob Then why do plenty of sources claim to know it?  
> **username_6**: @Starship there is currently great debate about the value of the Hubble parameter. Values from about 67 to 73 km/s/Mpc are quoted. Nobody knows what the value is to 2 significant figures, b  
> **username_1**: @ProfRob My point is why do people claim to know the value then? If you don't know something, why pick "a random value it could be" rather than saying you don't know?  
> **username_6**: @Starship "people" don't. The current uncertainties are about +/-2 km/s/Mpc at the 1 sigma level.  

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