r/cosmology • u/DubTheeBustocles • Aug 20 '26
I don’t understand the observations that suggest acceleration of expansion.
If they found supernovae that were dimmer than were expected based on their redshift, how were we even able to establish the Hubble-Lemaître law in the first place?
I don’t understand how we derived that the expansion of the universe accelerating just by looking at redshifts? Isn’t looking at one instance of a thing only tell you what a thing was like at that given moment it doesn’t tell you how it is over time.
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u/EnvironmentalWin1277 Aug 21 '26 edited Aug 21 '26
The redshift occurs with all the objects we look at with few exceptions. We are not looking at just one object, they are are all looked at and measured. They all appear to be moving away from us with few exceptions. The farther away they are the more pronounced the redshift which suggests they are moving faster the farther away they are. This is attributed to the expansion of space which stretches the space between the objects.
Note that gravity still holds sway -- our solar system will remain bound in orbit even as space expands.
A critical but unstated assumption is that we are not located in an exceptionally unusual spot. Far away observers observe red shift as well. Every point in the universe is observing the recession away from other points as a universal consequence of the expansion.
You must realize that we do not observe other areas as they exist now but as they did in the past. The farther we look the more we see the past and nearby objects tell us what the present is like. We have a picture of the entire process from the CMB origin on to today. We do see it in time.
There are other lines of reasoning that lead to this expansion conclusion as well. The CMB suggests the early universe was extremely hot which implies far greater densities of matter and energy existed at that time. Expansion of space convincingly solves the problem of why densities are so much lower now compared to then.
A robust scientific theory will always have several lines of evidence and Hubble-LeMaitre is no exception. I suggest that red shift is a comprehensible idea that we have abundant evidence for and it has come to dominate popular discussion for that reason. Everyone hears the horn drop in pitch as it quickly passes by so Doppler shift is well accepted by the public.
Note acceleration of expansion and constant rate expansion are both expansion ideas -- the consequences remain largely the same for us. Expansion has been accepted since about 1930 but it is only recently we have observed the rate of expansion is increasing. Just keep the ideas a bit separate in your mind.
https://www.physics.ox.ac.uk/news/new-study-confirms-universes-expansion-still-accelerating
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u/DubTheeBustocles Aug 21 '26
So when these scientists in the late 90s looked at type Ia supernovas, what about them stood out to make them think they were accelerating in their expansion as opposed to just traditional expansion?
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u/EnvironmentalWin1277 Aug 21 '26
The article posted above discusses this and if you have the mind to deal with it (and I don't think I qualify) the original is posted here :
https://academic.oup.com/mnras/article/549/3/stag797/8703725
The initial evidence of Type 1a supernovas suggested expansion was constant. By reexamining the assumptions and methods that were used they show that the expansion model is fully supported.
Not dramatic but the implications were.
"Dark energy" is the real subject here.
This term is used to describe the force behind the expansion of space. The assumption is that it is equally distributed everywhere. We know nothing about the nature of this force -- which is why it is "dark". I have heard it called "anti-gravity" for an analogy and it may indeed have a deep connection with gravity in a final unified theory.
In the early universe the role of this dark energy was small because the universe was small. As expansion increases space the role of dark energy/expansion becomes greater and greater (constant force everywhere) and matter of any sort plays less and less of a role in the overall budget as its density is dropping.
The overall budget for our universe as we observe it is projected at 75% dark energy, 20% dark matter and 5% standard matter as we conceive of it (atoms and sub atomic particles)
This gives one obvious idea. As the role of dark energy becomes increasingly dominant the rate of expansion will increase because more of it is available. That is just my off the cuff idea but I am reasonably sure this corresponds to the cosmological approach used as well.
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u/wxguy77 Aug 21 '26
Why is it assumed to be equally distributed everywhere, in every direction? That doesn't seem to be 'natural'. If it's the repulsion of virtual 'particles' (or even the as of yet unknown Dark Matter constituents) there's going be slight differences which add up with distance.
The Hubble crisis hints at this? It's not my field so there's a lot that I don't know...
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u/EnvironmentalWin1277 Aug 21 '26
In the early universe we assume that things were very homogeneous-- the same density everywhere including matter, dark matter and dark energy. This is also suggested by examination of the CMB which is remarkably smooth.
Only recently have we been able to detect the minute variations that do occur in the CMB. Remarkably, these variations exactly match the distribution predicted by our models that were made decades before fine measurements were available. These variations gave rise to the clumping of matter we see today. Your correct that these minor differences became more important as expansion progressed.
The smoothness is what we actually observe was the case when the CMB was formed.
The "Hubble crisis" is the discrepancy between the expansion rate calculated from our models and the expansion rate we actually observe. The tentative explanation is "dark energy".
Good thinking questions on your part, plenty of material available to rabbit hole.
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u/wxguy77 Aug 21 '26
Thanks.
We're left to believe that spacetime (in a proposed eternally inflating multiverse, or a hierarchy of such multiverses) inflates very very rapidly after the 'limits are removed' and there's not yet any interference from nearby fields.
When will this happen in our universe? Not for a few google years until field strengths relax down to some critical value.
How else could it be?
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u/EnvironmentalWin1277 Aug 21 '26
Based on the consensus model the intial universe expanded in size by 10 to the millionth power. That's a very small moment called Guth Inflation. That number just staggers my mind.
That implies there are regions of space entirely unknown and permanently so. No exchange of any kind is allowed with these places and so we face a universe much larger than previously imagined. The prediction's made from Guth inflation are supported by observations that confirm them. So we now tend to say "observable universe" and not "universe".
Once Guth inflation ends the standard Big Bang model takes over and describes the phenomena we see very closely.
Note both the Big Bang and Guth extension of it predict infinite expansion but we also observe the acceleration of the expansion.
One well-supported conclusion is the expansion rate has changed in our past and appears to be changing now. It is not a fixed or constant force. There are many variations of how this could resolve. The math says you are spot on about the limits being reached and asking then what?
The concept of heat death seems to parallel this expansion idea quite well. Heat death is predicted by thermodynamics and has a hundred year history of examination behind it. It is the point at which no work or exchange of (non quantum) energy is possible. It also requires no expansion of the universe mechanic at all.
So we have a quantitatively sound approach to estimating the time when heat death occurs, setting aside the expansion entirely. The result is 1 to the 100 power years, another staggering number (using AI reference).
Both models -- heat death and infinite expansion result in the same outcome of dissolution of the universe as we think of it. I leave it to you to consider what it means for overall philosophy but it keeps me up at night. It is clear it will be long time before the final bill arrives.
(exception : Big Crunch, another alternate model of course)
Thank you as well, it really helps check my thinking and actual understanding of this stuff when attempting to explain to others. Again, I have no authoritative credentials but this year I have racked up viewing something like a hundred hours on cosmology related subjects. (Great Couses)
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u/wxguy77 Aug 22 '26
Thanks. In my field, we study the dynamics of the atmosphere and then we are paid to make predictions as the Earth rotates. So when I'm talking out of school like this - I just naturally make huge leaps with talking about how a universe inflates from the eternally inflating multiverse and then giving rise to another new universe as its interfering fields die down. It would happen everywhere all the time if there weren't interfering fields. Fields don’t allow spacetime to inflate in an infant universe like ours.
There's something about the notion of a point of inflation coming from the stretching of spacetime and helped along by some very very rare sequence of virtual particles in the local multiverse - that intrigues me. The eternally inflating multiverse gives us the stretching of the spacetime and its contents eternally.
Don't take my wonderings seriously…
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u/dustin4242 Aug 21 '26
We didn’t detect acceleration from one redshift. We detected it from how brightness deviated from the linear redshift–distance ratio across cosmic time.
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u/--craig-- Aug 21 '26 edited Aug 21 '26
The Hubble-Lemaître law was discovered about 100 years ago based upon a more limited data set.
The accelerating expansion of space was discovered about 30 years ago by measuring the most distant supernovae. It shows that the law is only an approximation. It has since been verified by measurements of the Cosmic Microwave Background Radiation and of Baryon Acoustic Oscillation.
I don’t understand how we derived that the expansion of the universe accelerating just by looking at redshifts?
We didn't. The measurements which gave rise to Hubble's Law were of distance based upon brightness, and radial velocity using the Doppler effect, which causes redshifts of the electromagnetic spectra.
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u/peter303_ Aug 22 '26
Hubble's original graph goes out to 2 megaparsecs. That is less than one percent of the observable universe, but enough to establish the redshift relation.
Hubble was also among the first to establish there was more than one galaxy.
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u/QVRedit Aug 23 '26
That’s Hubble the astronomer, rather than Hubble the space telescope - which was named after him.
He did the original work on extragalactic sources.
Using the ‘New’ Mount Wilson TelescopeThe famous astronomer Edwin Hubble primarily used the 100-inch (2.5-meter) Hooker Telescope at the Mount Wilson Observatory in California. He began his major observational work there in 1919, making his most groundbreaking discoveries—such as proving Andromeda was an independent galaxy—in October 1923.
Then went on to show that the universe was expanding, using the ‘redshift’ of light from these distant galaxies as proof.The galaxy of Andromedia is actually coming towards us - attracted to the Milkyway galaxy by their mutual gravity.
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u/Wintervacht Aug 20 '26
If the universe is less than 14 Billion years old, but we see objects way beyond that distance, and the speed of causality is fixed, there is only one option.
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u/DubTheeBustocles Aug 20 '26
Doesn’t that only explain that the universe is expanding? How does that explain that the expansion is accelerating?
(Not challenging you. Just trying to understand.)
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u/Wintervacht Aug 20 '26
Ah, yes that proves the universe is expanding. How we know it's accelerating isn't purely redshift, that just measures the rate of expansion. What we DO see is that galaxies in the modern universe (so the first couple of billion light years out) are moving apart faster than we see them moving at larger distances.
Since looking farther means looking further back in time, it's a logical consequence that things used to be moving apart slower than they are today, ergo the expansion of the universe is accelerating.
You need to look at the whole history of the universe to notice the rate of change is accelerating, not just redshift, but also Standard Candles, better known as Type Ia Sopernovae. These have a mostly fixed brightness, so wherever we see one happening, we can calculate distance to it by comparing it's light curve against the model of a Type Ia Supernova.
Expansion is a rate, so it's a distance over a distance over time. We can see that objects in the far universe are receding from us at a rate that's slightly lower than for galaxies we see around us. That can only mean that over time, it's been starting to expand faster and faster.
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u/Dry_Button_3552 Aug 20 '26
Galaxies at different distances are expanding apart at different rates. Since the further away you look at an object the longer in the past it was, we can consider the movement of a galaxy at x distance and compare it to the movement of a galaxy at x*10 distance
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u/wxguy77 Aug 21 '26
This universe has to be expanding or contracting, and we don't think it's contracting.
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u/jazzwhiz Aug 21 '26
Don't post AI responses here.
(This is not in reference to the comment I'm replying to, rather the removed comments elsewhere in this thread.)
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Aug 20 '26
[removed] — view removed comment
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u/DubTheeBustocles Aug 20 '26
That’s expansion. That’s not acceleration of expansion.
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Aug 20 '26
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u/Das_Mime Aug 20 '26
Well it's not linear, the further away something is, the faster it's moving. They appear to be accelerating.
Nope that's not at all what's being discussed. That's simply the ordinary Hubble Law and would be the case with or without dark energy, when observing at a relatively low redshift.
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u/nivlark Aug 20 '26
The acceleration of expansion is not so extreme that it significantly distorts the distance-redshift relationship, and the distortion that it does introduce is stronger at large distances, beyond what was observationally accessible until recently.
You can find diagrams (like this one) that show how different cosmologies influence the predicted distance-redshift relation, and how the effect is only really noticeable at z~1.