<p>We know the universe is expanding because we see distant systems <a href="https://en.wikipedia.org/wiki/Redshift" rel="nofollow noreferrer">redshifted</a>, which implies that they are receding from us. We don't actually observe their distance to increase over time, as such changes are too minuscule to be observable over human time scales. We also don't measure distances in relation to the size of our galaxy. Normally we infer distance from redshift, and for independent distance measurements, we use the <a href="https://en.wikipedia.org/wiki/Cosmic_distance_ladder" rel="nofollow noreferrer">distance ladder</a>.</p>
<p>So the expansion of the universe cannot be related to any potential change in the size of our galaxy.</p>

CC BY-SA 4.0
2023-08-20T03:02:40.327Z
2023-08-20T03:02:40.327Z
Sten
<p>We measure the Hubble constant via the cosmic distance ladder and sometimes by cosmological doppler shift, the standard candle, which infers one's reccesional velocity by it's luminosity.</p>
<p>So here, the Tully-Fisher relation, which constitutes a rung of the cosmic distance ladder finds the relation  between the intrinsic luminosity and asymptotic rotational velocity or emission spectral line width (by long slit spectroscopy), and since the luminosity decreases as the apparent magnitude due to the inverse-square law, we can measure the Hubble constant, but via this if we already have the hubble constant we can also estimate the asymptotic rotational velocity and via Baryonic Tully Fisher relation, we can find the mass (this too depends on the balance and velocity so other methods would be better), usually galaxies maintain a balance of mass and centrifugal force, generated by the velocity, but if the mass is greater than the centrifugal force however (hypothetically) the galaxy would start contracting, and we can measure it with the Hubble constant</p>
<p>But it does not have a direct relation, because the dark energy, the cosmological constant can't effect such a <strong>dense</strong> lump of baryonic and dark matter full of gravity, and set it into expanding and contracting, instead via the Tully fisher relation we actually just measured the relation between luminosity and contraction/expansion, and then on the basis of the apparent magnitude we linked to the Hubble constant.</p>
<p>Note: We did not observe any galaxy that is contracting, but it is not impossible because if a larger galaxy is tidally interacting with the smaller one in such a way that it set's into contracting and possibly an irregular galaxy</p>
<p>Thanks!</p>

CC BY-SA 4.0
2023-08-20T05:00:20.590Z
2023-08-20T06:34:56.333Z
2023-08-20T06:34:56.333Z
Arjun
<p>How fast is our galaxy contracting?  Could our &quot;expanding universe&quot; measure actually be a measure of how fast our galaxy is contracting from other contracting galaxies. Thank you .</p>

<galaxy>
Galaxies contracting
Justathought
# Galaxies contracting

(Asked by: username_0 on 2023-08-20)

How fast is our galaxy contracting? Could our "expanding universe" measure actually be a measure of how fast our galaxy is contracting from other contracting galaxies. Thank you .

**Tags:** galaxy


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## 2 answers

### Answer by username_1 on 2023-08-20. Score: 3

We know the universe is expanding because we see distant systems [redshifted](https://en.wikipedia.org/wiki/Redshift), which implies that they are receding from us. We don't actually observe their distance to increase over time, as such changes are too minuscule to be observable over human time scales. We also don't measure distances in relation to the size of our galaxy. Normally we infer distance from redshift, and for independent distance measurements, we use the [distance ladder](https://en.wikipedia.org/wiki/Cosmic_distance_ladder).

So the expansion of the universe cannot be related to any potential change in the size of our galaxy.


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### Answer by username_2 on 2023-08-20. Score: 1

We measure the Hubble constant via the cosmic distance ladder and sometimes by cosmological doppler shift, the standard candle, which infers one's reccesional velocity by it's luminosity.

So here, the Tully-Fisher relation, which constitutes a rung of the cosmic distance ladder finds the relation between the intrinsic luminosity and asymptotic rotational velocity or emission spectral line width (by long slit spectroscopy), and since the luminosity decreases as the apparent magnitude due to the inverse-square law, we can measure the Hubble constant, but via this if we already have the hubble constant we can also estimate the asymptotic rotational velocity and via Baryonic Tully Fisher relation, we can find the mass (this too depends on the balance and velocity so other methods would be better), usually galaxies maintain a balance of mass and centrifugal force, generated by the velocity, but if the mass is greater than the centrifugal force however (hypothetically) the galaxy would start contracting, and we can measure it with the Hubble constant

But it does not have a direct relation, because the dark energy, the cosmological constant can't effect such a **dense** lump of baryonic and dark matter full of gravity, and set it into expanding and contracting, instead via the Tully fisher relation we actually just measured the relation between luminosity and contraction/expansion, and then on the basis of the apparent magnitude we linked to the Hubble constant.

Note: We did not observe any galaxy that is contracting, but it is not impossible because if a larger galaxy is tidally interacting with the smaller one in such a way that it set's into contracting and possibly an irregular galaxy

Thanks!


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