Is the expansion actually accelerating? Or does it just appear so?

  • #1
I pulled this quote from an article on the Hubble Constant:

"...for example, if the Hubble Constant was determined to be 50 km/s/Mpc, a galaxy at 10 Mpc, would have a redshift corresponding to a radial velocity of 500 km/s."

In this illustration provided from the article, if this was the measured observation from earth, this describes a constant rate of expansion, not an acceleration. It only appears to be an acceleration because of our somewhat fixed vantage in the universe. If the expansion is actually accelerating, we would expect to see velocity in addition to the constant every time you measured out another 10 Mpc (to stay within the framework of the provided example). So if the constant rate of expansion was 50 km/s/Mpc , an expansion that was also authentically accelerating would yield 50 km/s/Mpc +X ... with X equaling the velocity added by the acceleration of expansion. One number describing a velocity does not give any indication of acceleration, what is needed is a ratio that shows an increase in velocity over distance beyond what one would expect to observe from a constant expansion.

Hubble's constant is how we know the universe is expanding, but I am not clear how it shows acceleration, if it indeed does.

So... I already know that my observation is not common knowledge because it is never brought up in public explanations about the expansion or conversations about dark energy. There is often a mention of how the expansion is happening everywhere at once, and this is illustrated in several different ways to help confused people understand why it seems we are in the "middle" of the big bang... I get all that, no need to re-hash. What I am saying is, there is a corollary observation of "acceleration" to the expansion that is equally confusing. A universe expanding at a steady rate won't look much different from a universe expanding at an accelerating rate. An authentic acceleration may indeed be occurring, and perhaps it is simply not explained well, because no one has come up with good pictures to paint to describe the difference without thick equations (i'm math illiterate)... I dunno. That's what I'm trying to get to the bottom of.

What is the rate of "acceleration" and how are we differentiating it from the observed "acceleration" of distant objects we would expect to observe given a constant rate of expansion?
 

Answers and Replies

  • #2
Bandersnatch
Science Advisor
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Take a look at the Hubble diagram shown at 5:45 and listen to the explanation given over the next few minutes. See if it clears anything for you.
 
  • #3
Take a look at the Hubble diagram shown at 5:45 and listen to the explanation given over the next few minutes. See if it clears anything for you.
This is good stuff... it's gonna take me a minute to digest, but it is much more detailed than most of the similar productions I've reviewed.
 
  • #4
974
47
This may be a confounding between the words "observe" and "measure".

When the universe is said to be homogeneous, it is typically described as "looking the same everywhere". Clearly this can't be literally correct because the further one is looking the further in the past was what you are looking at - so with an expanding universe, the further the distance of the space you are looking at, the greater the density you see there, since the density of the universe was greater in the past.

What is really meant is that the observation is adjusted to describe the state of the distant space as it would look "now" if we were there locally to look at it. This adjustment takes into account the inferred passage of time, and is presented as a measurement - that the measured (taking into account these things) density of the universe "today, everywhere" is the same... this adjustment of observations into measurements in order to bring everything "up to date" gives us "cosmological time"... a snapshot of the way the universe is believed to be "everywhere, right now". That is what is meant by "looks the same everywhere"... how it would look after thinking about it and bringing it up to date.

When you watch the video, try to keep in mind that when cosmologists talk about observations of how things look, that is virtually always a shorthand way of meaning how things would be in cosmological time.
 
  • #5
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Just measuring the Hubble constant today doesn't tell you anything about the acceleration of the expansion, that is right. Luckily we can measure the Hubble constant in the past as well by looking at objects so far away that their light was emitted much earlier in the universe - their redshift depends on the expansion since the time of emission, which includes the expansion in the earlier universe.

If measurement uncertainties wouldn't be an issue we could watch the same galaxy over time, see how its redshift increases from year to year and determine the acceleration based on that. While ELT will be able to measure the expansion itself with that approach (if this project gets approved - it would take thousands of hours of observation time), it is not sensitive enough to accurately determine the acceleration of the expansion from that.
 

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