Hubble's Constant: Calculating Expansion Rate

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In summary, the rate of expansion/reduction in density of the universe can be calculated through various methods such as measuring the Hubble constant, which is currently estimated to be 70 km/s/megaparsec (with a 10% error). This value is not constant and can vary over time. The expansion of the universe is currently accelerating, but this does not necessarily mean that the Hubble constant is increasing. In fact, it is presently decreasing. The acceleration and the Hubble constant are two different things and an increasing acceleration with a decreasing Hubble parameter implies an equation of state with a state parameter w > -1. After a certain point, the cosmological constant will dominate the expansion of the universe, leading to a more
  • #1
K.J.Healey
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And the rate of expansion/ reduction in density of the universe.

I'm wondering if anyone has links to the physical research that has been done to calculate the rate at which distant, stars, galaxies, etc are moving away from us/each other.
Theres a million and one theories on how the universe is expanding, but I'm wondering if anyone has links/information of any actual empirical results, perhaps due to the redshift of distant galaxies.

I've searched the internet and its mainly results rather than data. Such as saying the HC is 70 km/s/megaparsec (10% error), but not actually giving the data on the distant galaxies they used to get this value.

Any help is appreciated! Thank you!

-KJH
 
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  • #2
Most of the results will be in published journals
I can't think of classic paper references of hand but anything by W.L. Freedman (http://www.ociw.edu/research/wfreedman/ [Broken]) should give you the answers
 
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  • #3
One of Ned Wright's pages gives links to papers on a number methods for measuring the Hubble constant.

I'm not sure this is all you want, though. The value of the Hubble constant is not constant in time, it's constant in space at any instant of time. The value of the Hubble constant that you quoted is its value now, which is different from its value in the past and future.

A common misconception is that, since the expansion of the universe is at present accelerating, the value of the Hubble constant is presently increasing. Actually, it is presently decreasing.

To figure all this out, one needs the scale factor as a function of time, i.e, one needs to solve the Friedmann equation, so I think you want to know how observations lead to a particular best-fit solution to the Friedmann equation. But I could be wrong.
 
  • #4
Thanks. For those also interest a good read on the current state of things is this paper:
http://www.blackwell-synergy.com/servlet/linkout?suffix=b11&dbid=64&doi=10.1046%2Fj.1468-4004.2002.43110.x&key=2001ApJ...553...47F
 
  • #5
So saying that the Acceleration of the universe's expansion is increasing, while Ho is decreasing would mean what? The rate of acceleration of the universe (is that called Jerk?) is changing negatively/slowing down?
 
  • #6
K.J.Healey said:
So saying that the Acceleration of the universe's expansion is increasing, while Ho is decreasing would mean what?
Increasing acceleration means that the second time derivative of the scale factor [itex]a[/itex] is increasing [itex]\ddot a > 0[/itex].

The scale factor measures the relation between a past cosmological distance and today for example a = 0 for t = 0 and a = 1 for today.

On the other hand, the decrease of the Hubble parameter means [itex]\dot H < 0[/itex]. If you apply the definition of the Hubble parameter [itex]H = \dot a / a[/itex] you get:

[tex]\frac{d}{dt} \left( \frac{\dot a}{a} \right) = \frac{a \ddot a - \dot a^2}{a^2} = \frac{\ddot a}{a} - H^2[/tex]

As you see these are two different things and [itex]\ddot a > 0[/itex] does not necessarily imply [itex]\dot H > 0[/itex].

Now, regarding your question.

If you insert the Friedmann equations without cosmological constant in the last relation, at least for a flat space k = 0 you can easily show that an increasing acceleration with decreasing Hubble parameter implies an equation of state with state parameter w > -1.

I.e., if the universe contains some dark energy with w = -1, then there still exists a non negligible amount of matter so that the expansion still does not get exponential (with the definition of the Hubble parameter I gave you above you can show that H = constant means a -very fast- exponential expansion a ~ exp Ht).
 
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  • #7
George Jones said:
A common misconception is that, since the expansion of the universe is at present accelerating, the value of the Hubble constant is presently increasing. Actually, it is presently decreasing.

I have done an integration of the Lambda_CDM Friedmann equation with present best values and was surprised to learn that the Hubble constant H will decrease by about 10 km/s/Mpc for a doubling of the expansion factor. After that H remains more or less constant, since the cosmological constant then dominates the expansion curve completely.

Does this sound more or less right?
 
  • #8
You result is correct. For expansion factor a = 2 (with a = 1) today, the Hubble parameter is H = 0.87 Ho (Ho today = 71 Km/s Mpc).
 
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What is Hubble's Constant and why is it important in calculating the expansion rate of the universe?

Hubble's Constant is a measure of the rate at which the universe is expanding. It is important because it helps scientists understand the age and size of the universe, as well as its future expansion.

How is Hubble's Constant calculated?

Hubble's Constant is calculated by measuring the redshift of light from distant objects, such as galaxies. This redshift is then used to determine the object's distance from Earth. By comparing the distance to the object with the speed at which it is moving away from Earth, Hubble's Constant can be calculated.

What is the current accepted value for Hubble's Constant?

The current accepted value for Hubble's Constant is approximately 70 km/s per megaparsec. However, there is ongoing debate and research to refine this value.

How does Hubble's Constant support the theory of the Big Bang?

Hubble's Constant supports the Big Bang theory by showing that the universe is expanding at a constant rate, which can be traced back to a single point of origin. This supports the idea that the universe began with a massive explosion, as described by the Big Bang theory.

What are the potential implications of a changing value for Hubble's Constant?

A changing value for Hubble's Constant could have significant implications for our understanding of the universe. It could indicate that the expansion rate of the universe is not constant, or that there are other unknown factors affecting it. It could also challenge the currently accepted theories of the universe's origins and evolution.

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