Submitted by **melanthius** t3_zu2329
in **space**

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Submitted by **melanthius** t3_zu2329
in **space**

[removed]

Have you considered cosmetology?

Or Astrology?

Or astrocosmetology?

With a minor in Geology… *errrr* I mean, um, “Crystal Power”

I think the numbers for saraswati are helpful, but I was trying to ignore the effect of andromeda approaching Milky Way. What would it be like for some object much closer than saraswati but one that is not approaching us specifically?

Is andromeda blueshifted then? Edit- I see now that it is from a google search. That makes it a bad example for this

The constant for the universe expansion is (70km/s)/Mpc, so if we approximate MW-Andromeda to be 1 Mpc, in a million years it would be ~233 light years further. In real life, thought, it has a relative speed towards us and the gravitational bound between our galaxy and Andromeda is enough to render the expansion useless

Computing distances in cosmology is a bit of a pickle since within relativity there are several different measures of distance all of which have a different physical meaning. This is a direct consequence of relativity. Some of the types of distance you could compute are physical distance, comoving distance, angular diameter distance, and luminosity distance as detailed here https://en.wikipedia.org/wiki/Distance_measure .I will move on and assume that you want to compute the "proper distance" since that is probably closest to what you would conventionally call a "distance" based on the phrasing of your question. As shown in the article I posted above the proper distance is related to the comoving distance as follows: d=dh*a , where d is the proper distance, dh is the comoving distance which ignores the expansion of the universe and is constant in time and "a" which is the scale factor which is an increasing function of cosmological time and is given by the first Friedman equation https://en.wikipedia.org/wiki/Friedmann_equations .

Check out this link. https://jila.colorado.edu/~ajsh/courses/astr2010_22/evol.html the first plot gives you the growth of the scale factor as a function of physical time. Of course the evolution depends on the cosmological model you consider hence the different colored plots. The canonical cosmological model that we currently believe to be correct is the purple line in the plot hence you should focus only on that. If you look at the horizontal axis, it is labeled in "billions" which implies that the growth of the scale factor is extremely tiny, hence the distance would barely be altered (see below). If you choose a billion years the scale factor increases by about 10%, hence, in a billion years both points you mentioned above will be approx 10% further away in proper distance.

In case you are semi-educated in calculus, you should be able to recognize the first Friedman equation as being a simple first order differential equation for function a. By solving this equation with initial condition a(t=0)=1 you should be able to find a(t=1 million years). We need a few more elements before we do that. We need to specify the cosmological parameters that enter the 1st Friedman equation. We set k=0 because as far as we know our universe is devoid of curvature (it is flat) and then on the right hand side we assume that the first term scales as 1/a^3 because it represents the dark matter and the second term is constant (dark energy). We also need to ensure that evaluated today, the first term is 1/2 of the second one roughly speaking since there is twice as much dark energy compared to dark matter in the present moment. You will also need the critical (total) energy of the universe from https://pdg.lbl.gov/2020/reviews/rpp2020-rev-astrophysical-constants.pdf . If you put everything together you find the scale factor for the two times you mentioned above.

a(t= 1 million years)= 1.00007

a(t= 1 billion years)= 1.07

This is essentially a full explanation of how to derive the purple line in the plot. Now that we have the scale factor all we have to do is to multiply it by the distances you mentioned

Andromeda after 1 million years will be about 0.007% light years further away and about 7% in 1 billion years. Of course that is ignoring the "peculiar velocities" which is the technical term of what you mentioned above as "local" velocities

Here's a good one for you then to show one million versus one billion.

If you count each second as it passes, you will reach one million in eleven days.

If you count to one billion it'll take 31 years. (31 years is also one thousand spans of eleven days).

So 2.5 M is around 28 days and 4 B is more like 120 years.

I heard someone say the other day "a billion minus a million is about a billion."

1000 minus 1 is almost 1000.

It's every bit as valid as that statement.

10 minus .01 is almost 10.

That too. You're taking away a thousandth.

Yep. Puts it in perspective.

I don’t think this has anything to do with the question

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The universe’s expansion works at distances over 250 million light years. Less than that, and gravity is generally stronger. More than that, and the expansion is close to 7% per billion years.

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Hello u/melanthius, your submission "Hoping to put hard numbers on the universe’s expansion to put it in perspective" has been removed from r/space because:

- Such questions should be asked in the "All space questions" thread stickied at the top of the sub.

Please read the rules in the sidebar and check r/space for duplicate submissions before posting. If you have any questions about this removal please message the r/space moderators. Thank you.

Current evidence is that the universe is expanding at faster that the speed of light.

It is poorly understood (especially by me) at this point.

earthman34t1_j1grbww wroteThese things are not trivial to calculate at universe-spanning distances where relativistic effects and universe expansion become factors. Doing some simple math and ignoring these factors, at it's current velocity based on measured red shift, the Saraswati supercluster would be about 280,000 light years farther away in a million years, and 280 million light years in a billion years, although that later figure would be off by an increasingly large factor. The Andromeda galaxy is moving towards our galaxy at a much lower velocity, so in a million years it would be around 1000 light years closer, and in a billion years about a million light years closer.

FYI I am not a professional astronomer or cosmologist.