# What is the evidence for an accelerated expanding universe

1. Mar 12, 2015

### wolram

Is the red shift evidence enough to define accelerated expansion

2. Mar 12, 2015

### CalcNerd

Yes.

While Professor Hubble did find some galaxies were drifting towards us, most were moving away. And the farther these galaxies are from us, the faster they were moving away as determined by their red shift. Hence proof of an expanding universe.

3. Mar 12, 2015

### bapowell

The OP is asking about *accelerated* expansion. One line of evidence (from type 1a supernovae) is the distance-redshift relation.

4. Mar 12, 2015

### Garth

CalcNerd you have confused expansion and acceleration.

Hubble's findings, first interpreted by Monseigneur Georges Lemaître, indicated an expanding universe.

This expansion was expected to decelerate because of the attractive gravitational forces of the matter within the universe in accordance with the cosmological solution of Einstein's GR field equation.

In 1998 two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team each used Type Ia supernovae as standard candles and showed these SNe were dimmer than expected at that red shift (z) under the old GR model. This suggested the universe had accelerated in its expansion rather than decelerated.

Such an acceleration can only be explained by the addition of a Cosmological Constant $\Lambda$ or Dark Energy (DE), in which p = - $\rho$c2 or so.
The density of the DE thus required exactly makes up the density of the universe to its critical value which results in a spatially flat universe. As CMB observations indicate the universe is flat this seems to confirm the observed acceleration and inferred DE. Other observations also indicate the presence of (cold) Dark Matter (DM) in the universe.

These are major components of the standard $\Lambda$CDM model of cosmology.

However this standard model requires the existence of DE and DM, which have not otherwise been detected in the laboratory, and it also requires the assumption that SNe Ia are standard candles out to cosmological distances.

wolram was asking whether the acceleration could be defined by red shift alone.

wolram, the answer is no - you have to have something else by which to measure the distance of an object observed at a red shift z.

The Perlmutter, Riess and Schmidtet et al discovery used SNe Ia as standard candles to feed M into the cosmological distance modulus, the result seems to be confirmed by baryon acoustic oscillations (BAO) observations and the DM fits in well with large scale structure formation.

(1 + z) gives you the amount the universe has expanded by since the emission from the object at 'z', and when compared with distance gives the scale factor

There may yet be an Age Problem in the early universe as discussed here a few times!!

To confirm the model further we badly need other distance measures.

(Crossed with bapowell)

Garth

Last edited: Mar 12, 2015
5. Mar 12, 2015

### CalcNerd

My mistake and I should have realized that something was amiss due to the status of the OP. And maybe I need to learn to read a bit more carefully.

6. Mar 12, 2015

### Chalnoth

Like most things, it's a combination of observations. The primary pieces of evidence are supernova surveys, baryon acoustic oscillation observations, cluster surveys, estimates of $H_0$ from relatively nearby, and the CMB.

The combination of the CMB and BAO data in particular give the large-scale geometry of our universe as well as an accurate determination of the early expansion rate and density. The combination of these is good evidence that there is some other energy density (or cosmological constant) in our universe. Measurements of the nearby expansion rate ($H_0$) give another check on the geometry (with the CMB data alone, one could conceivably fit the data with no dark energy and an open universe, aside from one caveat mentioned below, but the current expansion rate would be wildly different).

The supernova observations give the most direct evidence of accelerated expansion, as they directly measure the redshift-distance relationship, which is a derivative of the expansion rate over time.

Cluster surveys serve as a consistency check, as they provide accurate estimates of the matter density of our universe.

Finally, the CMB has a rather subtle effect at large angular scales: the Integrated Sachs-Wolfe Effect. This effect occurs because if you have no dark energy, gravitational potentials are constant over time (in the linear approximation, which means very large scales). But with dark energy, gravitational potentials slowly decay as the universe expands. As light goes into a potential that is decaying, it falls into a deeper well than it exits out the other end. So it picks up a slight blueshift. The reverse happens as a photon travels through a void.

Overall, this adds a slight additional variation to the CMB at large scales. The CMB data definitely fit a universe with the ISW effect better than one without, though I'm not sure off the top of my head out significant that detection is (it may not be all that strong of a detection, as large angular scales have intrinsically large error bars).

Last edited: Mar 13, 2015
7. Mar 13, 2015

### Quds Akbar

Yes, the space between us and galaxies might be expanding. And some galaxies are moving away from us faster than the speed of light because the space in between us and them expands and the speed limit does not apply to the expansion of space itself.

8. Mar 13, 2015

### bapowell

The expansion rate is not a speed. Superluminal recession velocities are admissible because special relativity applies only locally in the universe: two receding galaxies (comoving with the expansion) are in separate inertial frames.

9. Mar 16, 2015

### slatts

The difference between acceleration and non-accelerated motion is often neglected by people interested in physics, but The Stanford Encyclopedia of Philosophy (available online) points out the fact that, in the view of the philosopher and physicist Sklar, acceleration is the most mysterious component of relativity.

10. Mar 16, 2015

### bapowell

Accelerated expansion of the universe is not related to accelerated observers in special relativity.

11. Mar 16, 2015

### slatts

Just so I can understand your point better (-and I apologize for mine having made no sense at all), if the galaxies were all moving as the result of some common impulse like a Big Bang, is it differences in the curvature of their paths that would have left them in separate inertial frames, and, if so, would those differences have had to result from quantum perturbations? (I had not understood that travel faster than light is prohibited only by Special Relativity; I had thought it applied in GR as well, and that the distinction between expansion and travel had been the ruling factor.)

Last edited: Mar 17, 2015
12. Mar 17, 2015

### bapowell

First, don't think of galaxies moving as a result of some impulse. The expansion is not due to some force created at the big bang, rather the expansion of space is a consequence of the gravitational dynamics of uniform, isotropic space. In fact, the universe could just as easily be contracting (mathematically speaking, it's a choice of initial conditions).

The question of inertial frames has to do with the expansion: special relativity only applies locally where the effects of expansion are negligible and inertial frames can be found to a very good approximation. While we can safely define inertial frames here on Earth, and we can likewise probably do so on a planet in Andromeda, these cannot possibly be the same inertial frame.

According to the inflationary universe paradigm, quantum fluctuations in the early universe generated the initial density perturbations that would later grow to form galaxies and galaxy clusters.

13. Mar 19, 2015

### slatts

What you're saying is consistent with the threads about how the Big Bang happened everywhere at once, and with what Sklar (who's sort of my idol because I was FINALLY able to get what feels like a shaky but intuitive grasp of relativistic time distortions and length contractions, with the help of the diagrams in his 1970's book) said about General Relativity being based on experimental data that wouldn't necessarily hold everywhere. (Your "probably...in Andromeda" is the FIRST reference to even a HYPOTHETICAL version of such data that I've seen in print ANYWHERE else, so I offer my tentative thanks for the reality check on that; I was beginning to think that maybe Sklar was kidding.) I've also googled enough of your terminology to realize that "initial conditions" can have a purely mathematical significance, although I'm figuring that cosmology may tend to use them a lot in relation to a physical differentiation between attractive and repulsive gravity.

Before taking that issue (-which interests me a lot) further, though, I've got to ask something about views on Special Relativity that Brian Greene presents on pages 136-139 in the 2004 edition of The Fabric of the Cosmos, and elaborates with equations and a diagram in his notes on its p.504-505. He attributes, to S R, significant effects on the simultaneity of events in "a nearby galaxy" that result from infinitesimal changes in the local velocity of an observer here. (It's illustrated through "bread slicing" analogies with the relation between motion--particularly its direction--and spacetime events, but the details in his notes are very specific.) Since Andromeda's the galaxy nearest our own, he plainly sees them as being in the same inertial frame, and you don't see that even as a possibility. Is this due to Planck Satellite data since 2004, or is Greene sort of stretching things, or am I just missing something here?

14. Mar 19, 2015

### Drakkith

Staff Emeritus
How can they be in the same inertial frame when they are moving relative to one another?

15. Mar 19, 2015

### Staff: Mentor

He's not describing our actual universe in this thought experiment; he's describing an idealized universe in which SR is valid even on large distance scales. In our actual universe, it's not. For the purposes of his thought experiment, the difference doesn't matter; but the fact remains that it's an idealized thought experiment only.

16. Mar 19, 2015

### slatts

(This crossed the reply by PeterDonis:) I understand that they are moving relative to each other (-in fact, I think I've heard that they're heading slowly toward a collision), so I'd figured that where Greene and Bapowell might be disagreeing is on whether Special Relativity applies to events in different inertial frames.
It's possible, alternatively, that Greene has (or had) a much broader view of the approximations adequate for the consideration of objects as being in the same inertial frame.

Greene's book's not free online, but the example, illustrated in its notes with "the usual concept of spacetime diagrams [italics his] taught in courses on special relativity" and including a relevant example of one, shows how a change in the direction taken by an observer walking (yes, walking) on a planet in Andromeda changes those earthly events which are simultaneous with his walk from events that are decades old in earth's past (when the observer's walking further away from earth) to events decades away in earth's future (when he's walking toward earth).

17. Mar 19, 2015

### slatts

Thanks.

18. Mar 19, 2015

### FRK

the red wavelength indicates that light is going away from us but on large scale it becomes game of time .if something in definite room goes away from us doesn't means that always room is expanding it might be the acceleration of object otherwise something unknown is working.am not sure about anything also not proving wronge anything because the expansion of universe is not totally proof.

19. Mar 19, 2015

### slatts

Last edited: Mar 19, 2015
20. Mar 19, 2015

### slatts

Since I can't find any sign that Greene either referred to the effects of SR on which spacetime events in one inertial frame are simultaneous with any particular spacetime event in another as thought experiments or intended for his description of those effects to have the effects such experiments might have had on his readers, is it possible that what he was describing is a close approximation to what happens in GR? Although Greene's observer on earth is identified as his reader, the Wikipedia article "Relativity of Simultaneity" says (with my italicization of some of its contents), <<The Lorentz-transform calculation...uses a definition of extended-simultaneity (i.e. of when & where events occur at which you were not present) that might be referred to as the co-moving or "tangent free-float-frame" definition. This definition is naturally extrapolated to events in gravitationally-curved spacetimes, and to accelerated observers, through use of a radar-time/distance definition that (unlike the tangent free-float-frame definition for accelerated frames) assigns a unique time and position to any event.>> I totally lack the calculus needed to form any judgment involving radar-time, so, does anyone familiar with the concepts involved think this might be a possibility?

Last edited: Mar 19, 2015