# Is the expansion of the universe really accelerating? Or does it just look like it?

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 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?

• The answer here is relevant: < physics.stackexchange.com/q/24337 > – N. Steinle Sep 9 '18 at 21:32
• Just because nobody brings it up at the popular level doesn’t mean nobody knows it. The effects you talk about are all laid out and explained in even the most basic intro cosmology textbooks. As a layperson, you are exposed to perhaps 0.001% of the full story; what you’re saying is very far from new. – knzhou Sep 9 '18 at 21:36
• If you really don’t want to see any equations, then all I can tell you is that the effect you’re talking about is universally accounted for already. It’s like asking if a car manufacturer has accounted for gravity. – knzhou Sep 9 '18 at 21:38

## 1 Answer

This seems to be the main problem you have :

A universe expanding at a steady rate won't look much different from a universe expanding at an accelerating rate.

And the answer to this depends on a key point : light travels at a constant speed and, on the scale of the universe, a speed that lets us see into the past.

When you look at very distant objects in the universe you are seeing them in the past. Say an object appears to be 5 billion light years, then the light we saw it by took five billion years to reach us and the object has moved since then.

We can tell they are moving because we can measure the wavelengths of certain lines in their spectrum and we know the value they would be if they were at rest and any difference tells us the velocity of the objects relative to us.

And from that we see a very interesting thing : the further away objects are (on a cosmic scale) the faster they are going. This data is consistent in all directions and gives rise to Hubble's Law.

That tells us that there is an acceleration going on and it relates to the distance objects are from each other.

So we reached the point of explaining that from our "snapshot" of the universe we see now, we can tell not only that it is expanding, but that there is an acceleration of that expansion.

Now you don't want mathematics, so all I can do is tell you that after the general theory of relativity (GR) was discovered a very significant theoretical finding was made called the FLRW metric. This metric predicts something non-relativistic physics cannot - that the universe can expand in the way we see.

More recently we have discovered that the universe is not simply expanding, but that the acceleration is faster than our early theory explained. This has led us to consider that something called "dark energy" exists that "super accelerates" (for want of a better expression) the expansion.

• I'll ask a natural follow-up to this: Since farther objects are necessarily seen at earlier points in cosmological time, how are we currently lining up distance/age/velocity? In other words, an object 10 billion light years away is measured to be traveling at a velocity X, but that's also its speed from that long ago. In an accelerating universe it seems the farthest away objects would also have the SLOWEST velocities since we're seeing them at the earliest time. But this is never how it's reported in popular articles. – JPattarini Sep 9 '18 at 23:19
• You are basically thinking of space-time as not being linked. To understand this you have to allow for time and space being intimately linked. You're also forgetting that the expansion works both ways. After the light we see left the objects it came from, space kept expanding and we (in effect) got further away and moved faster from the object (in relativity who is stationary depends on who is asking :-) ). I'm trying to keep this over-simplified, but really only the maths makes sense of this - common sense breaks down because it doesn't include GR. – StephenG Sep 10 '18 at 0:03
• @StephenG: You're missing the fact that the earlier theory predicted a deceleration, not just a slower acceleration. – D. Halsey Sep 10 '18 at 22:37
• We may be quite sure that the speed of light is constant across space, but we cannot be anything like as sure that it's constant over time. – Alan Gee Dec 5 '18 at 23:34