View Full Version here: : Gravitational Lensing and the accelerated expansion of the universe
17-04-2012, 04:51 PM
Interesting article here about a new study. The authors have looked at the statistics of gravitational lensing of quasars and shown that they are consistent with the current view that we live in a flat universe whose expansion is accelerating due to dark energy.
I particularly like the comment at the end from Professor Lineweaver about the importance of reaching the same conclusion from different methods. In a complicated universe where all experiments and observations are imperfect, it is really this that gives scientists confidence in theories, not a single perfect result.
And now for the rant...
Unfortunately, a certain type of person (rare in these esteemed fora but common in the world) seems unable to grasp this, but is forever insisting that a few odd points on a noisy graph of a complicated phenomenon means the whole theory has to be thrown out. Harrumph.
22-04-2012, 06:56 AM
I concur with Dave that one or two studies can't invalidate a line of thinking established my many independent lines of study. In reading the ABC article I was impressed by the following summation:
Professor Charlie Lineweaver, a cosmologist from the Australian National University, wrote "There are five ways to determine how fast the universe's expansion rate is accelerating." . . . "Schmidt used supernovae, which is still the most accurate method. There's also baryonic acoustic oscillations which are density waves that propagate through the universe, there's the cosmic microwave background radiation and you can also count the numbers of star clusters at given distances."
Understood, but the reference to baryonic acoustic oscillations is a new one on me. Can anyone shed light (so to speak)?
Is the term related to the following which appeared in Swinburne Magazine December 2011 Issue
Getting the universe’s measure:
"To assess the distances between galaxies as they move further apart, the team took advantage of the known ‘preference’ of galaxies to operate in pairs about 490 million light years distant from one another. This provides a useful ruler for comparing other cosmic distances."
This also was a new one on me. Help, anyone?
Dana, Weltevreden SA
23-04-2012, 11:41 AM
My understanding of baryon acoustic oscillations is that these are variations in density of matter across the universe. When you look at the cosmic microwave background, these are the 'lumpiness' you see. The size of these lumps (way back then) is related relatively simply (depending on what you think is simple) to the dark energy, or rate of expansion. They are also related to the distribution of galaxies today. One upshot of the size of these lumps is the 490mly scale.
I'd be interested to hear from anyone who understands this better than me, particularly if I'm completely wrong.
23-04-2012, 12:59 PM
Charlie delineates the BAO and the CMB so I suspect he is talking about the BOSS experiment at the SDSS, which reconfirms this data. This is of course evident in the CMB but the BOSS data does doubly and agreeably confirm this data.
I guess this is all about degree of confidence. Of course it presupposes that the basic physics applies at all times and at all places, but there have been a number of experiments to test this hypothesis and all have been positive at this time. Still we can never "Know" this to be true but simply have a high confidence based our analysis of the photons we receive.
The veterinarian's view (ie: my assumptions on the physics of astronomy may very well be very wrong)
In the beginning (not trying to sound biblical) there was a singularity which was highly ordered (very low entropy). 1 second later there was a massively expanding universe which now had a much higher entropy. Since the 2nd law of thermodynamics dictates that entropy can only increase over time, would that not mean that the universe can only expand as time passes? (This is perhaps where my biologist's view may come unstuck). I know that this is not directly concerned with acceleration of the universe, but it has just always struck me that in the days when it was believed that the universe may eventually stop expanding , or even begin to implode/shrink, wouldn't the 2nd law of TD have been there saying, hey dudes, entropy always increases and therefore a shrinking universe appears unlikely/impossible.
If I'm wrong I'll go to work and spey a dog
25-04-2012, 10:12 PM
A 2005 paper Eisenstein (http://arxiv.org/abs/astro-ph/0501171v1) gives a relatively straightforward explanation of baryonic acoustics once one makes the mental gear change from thinking in observational astronomy space to redshift space. My eyebrows went up at the statement, “It does rely on the well-understood linear perturbation theory of the recombination epoch to relate the perturbations in the photons . . . we have measured the relative distance between two radically different redshifts using a purely geometric method and the same physical mechanism” because the evidence set forth in the discussion didn’t seem to support a purely linear perturbation. I wondered whether supersonic shock behavior manifest itself in measureable turbulence. Correct me if I‘m wrong, but is the answer found in this point from the same paper: “the beat frequency between the peaks and troughs of the acoustic oscillations is a very small wavenumber that is well inside the linear regime.”
Eisenstein’s 2006 (http://arxiv.org/abs/astro-ph/0604361) paper addressed the Swinburne “galaxies operate in pairs 490 million ly apart” point this way: “We argue that the dominant non-linear effect is the differential motion of pairs of tracers separated by 150 Mpc. These motions are driven by bulk flows and cluster formation and are much smaller than the acoustic scale itself.” Other useful papers are: Percival et al. (http://iopscience.iop.org/0004-637X/657/1/51), and what is perhaps the last (or anyway latest) word on the subject, the 29 Mar 2012 Beth Reid et al. (http://arxiv.org/abs/1203.6641) paper. The arXiv list of Eisenstein’s papers (http://arxiv.org/find/astro-ph/1/au:+Eisenstein_D/0/1/0/all/0/1) is also worth a look. It’s heartening how many people are active in this field.
26-04-2012, 10:25 AM
The idea of starting from a singularity is an old idea that was dismissed some time ago. I would recommend Lawrence Krauss "A universe from nothing" as a good read which will adress your issues, or if you want the short story he has several 1 hour lectures on this subject on utube.
26-04-2012, 12:36 PM
Aah, entropy. A mind-bending topic.
Here's the out-of-date physicist turned banker's view.
What you're suggesting isn't wrong, but I think it's not exactly how most physicists would put it.
Firstly, the 2'nd law of thermodynamics (which is essentially about entropy) is not really a law like most physical laws. It's a statistical law which is an inevitable consequence of fundamental physical processes and the laws of probability. In fact it can be violated from time to time, it's just that the chances of a violation in any decent-sized system are negligibly small. Think about a chamber of gas, and the chance that (briefly) all the particles turn out to be in the left half of the chamber. It would be a decrease in entropy, it could happen, and the chances of it happening are so close to zero you can forget about ever seeing it.
So, in your contracting universe, it's not actually clear that the fact entropy would increase is a problem.
But, physicists seem to generally view the 2'nd law as about the most fundamental there is, and the universe seems to go to a great deal of trouble to not violate it. (Hawking's work on the entropy of black holes is a good example.) As a result, one line of thinking people have gone through is to look at the directionality of time. This pulls together a few observations:
Underlying physical processes are generally 'blind' to whether time is going forwards or backwards;
Nevertheless, we perceive time as going in a particular direction;
The 2'nd law provides a directionality for time - it's the direction in which entropy increases;
When we remember things, we increase entropy - that's why we remember the past not the future;
Then the big leap - Someone observed that the expansion of the universe can be thought of as a second directionality for time. As time moves forward, the universe expands, which coincides with entropy increasing. So - if the universe started contracting, would time appear to reverse?
Personally I think the last point reads way too much into the coincidence, however much smarter people than me think there is a real point here.
Finally, I mentioned the trouble the universe seems to go to to preserve the 2'nd law. Is that why things are arranged so that the universe will expand forever?
Anyone else more up to date on this?
26-04-2012, 01:13 PM
I suspect that the issue of time, its direction and its epistimological foundations exist on the boundaries of physics and metaphysics. Most of the contemplation of this issue seems to eminate from Departments of Philosophy rather than Departments of Physics per se. Perhaps this is part of a more general portent to the future as we enter a period where real measurement and verification is becoming extremely difficult and mathematical abstractions like string theory and quantum loop gravity will continue to suffer from a lack of experimental gravitas.
Further, saying "the trouble the universe seems to go to ..." opens up a philosophical minefield as well. We seem to be faced with the essential truth that any minor deviations from the key settings of the universe we find ourselves in would result in not only potential disagreements with the laws of thermodynamics but also unable to support the development of galaxies, stars, planets or life for that matter. So this begs the question "Did the universe go to the trouble ...?" contemplating the quintessential guiding influence or is this a simple anthropic quirk of fate in that this is the only universe in which we can find ourselves. Here again physics transitions to metaphysics.
I think Krauss has a pretty good stab at outlining the current state of play without transgressing the metaphysical barrier to far.
Frankly, until we have some view of what Dark Matter and Dark Energy is really all about then I believe these will continue to be murky waters.
26-04-2012, 01:26 PM
Maybe, but it got a good airing in the School of Physics in may day.
Again, absolutely agree, however this language seems well-used by reputable people as a figure of speech.
26-04-2012, 01:46 PM
My son dug up this recent TED Talk on a quite related range of topics the other day:
However, on the topic of asking the right questions, I couldn't quite stomach Brian's mentioning of Kepler, after all Kepler did investigate the laws of planetary orbital movement with some success…
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