Accelerating Galaxies

Photon_2013

In Cosmology books and tv programs it is mentioned the galaxies are accelerating and running away from us. My question relates to this observation.

When they are looking through telescope to the edges of visible universe, they find extremely bright supernova or "celestial candles", calculate its speed and then find the acceleration. But those are billions of light years away. So what they are looking at is the position and speed of that supernova as it was billions of years ago (since light took that much time to reach us). All they can say is those galaxies were accelerating billions of years ago. They don't know where those galaxies are now, so no way to tell if they continued acceleration, slowed to static speed or started collapsing.

For example if you are obseverving some galaxy 10 billion light years away, you are looking so far into the past it takes you closer to big bang (13.7 billion). That time, of course, you would expect the universe to be much different and show strange things. How can somebody know what is happening right now. If they cannot know, how can they use this observation to postulate presence of dark energy (right now)?

Thanks in advance.

Chronos

Determining the expansion rate of the universe is a difficult process. You need an independent measure of the recession velocity and distance to the object of interest. The first part is easy - recession velocity. A highly accurate determination of recession speed can be obtained by measuring redshift. The second part is not so easy - determining the distance. Standard candles is how this is accomplished. A standard candle is simply some feature of the object that is of known luminosity. Any inaccuracy will produce errors that can be highly significant. A complicating factor is you are forced to make certain assumptions about the nature of the object, the physics involved, effects of certain types of dimming that may exist, and so forth. One of the 'best' standard candles is type Ia supernova. They are extremely luminous and the stars that produce them are believed to be a very specific mass at the time of detonation and is predominantly produced by carbon ignition. Their peak luminosity is therefore believed to be extremely consistent - within 5% or so. It is on this basis the discovery of accelerated expansion was deduced. Imagine a series of photos taken of a bicyclist traveling away from you and the only measurements available were its doppler shift and the brightness of a tiny light it carried. We could be highly confident of the doppler shift reading, which tells us its instantaneous velocity. Now lets suppose the intensity of the light varied due to intervening patches of fog and fluctuations in the battery power output. That is why these measurements or so tricky. If the pictures are of different bicycles taken at different times, we cant even be sure the light bulbs are identical. Some bicycles may have led's, others standard incandescent bulbs, and some have krypton bulbs. Even though we know the velocity of each bicycle with great accuracy, we are much less certain of their true luminosity. Drawing any conclusion about the distance velocity relationship of all bicycles would be a daunting task. Fortunately, we have other, albeit less precise, tools to give us some assurance we are in the right ballpark. We can, for example, assume all bicycles are of similar size and use that to humor check our inferences regarding luminosity - which is how cosmologists approach this problem. It is this process which resulted in a distance luminosity relationship that indicates the universe is not only expanding, but, is accelerating. The work done by Perlmutter and Riess [conducted independently] was so thorough and compelling it is widely accepted by the scientific community today. Bear in mind all of these galaxies were of varying age with respect to the age of the universe so the measurements are representative over a wide range of time.

phinds

Photon, is some ways, the most important part of Chronos's response, and what you need to get clear about, it the last sentence.

If there were just one reading of one galaxy at 10B LY then your concern would be warrented, but a set of time-slices of the observiable universe gives resulsts that are HIGHLY consistent with the concept of an accelerating universe.

Photon_2013

Thanks Chronos. I am in complete agreement with you about how these calculations are done and how difficult they are. My understanding is that only when they started observing far away galaxies (hubble telescope), they found this expansion is accelerating.

I had emphasized parts of my post to indicate what the fundamental question is. It has to do with the fact that you are looking very, very far back in time. You can see some candle 10 billion light years away and use sophisticated means to calculate its acceleration etc. But that has nothing to do with what exists right now. The only thing you know is 10 billion years ago there were galaxies accelerating away from us. Ok, fine. But that is past so far away even solar system or sun did not exist at that time.

Just as a side note, the behaviour of galaxies close to us is different. Andromeda is on collision course with us.

Bandersnatch

To find out what happened to a galaxy 10bln ly away during the time it took its light to reach us, look at the galaxies closer to us.
Any one 9bln ly away is still receeding, so nothing much changed* in the intervening billion years. The ones 8bln ly away - still receeding. So are the ones 7, 6, 5 bln ly away, and so on. This tells you that the recession did not stop 10bln years ago, but likewise continued 9, 8, 7 and so on years ago, throughout the history of the universe.

So unless you deny the validity of the cosmological principle, then you might safely say that you know what happened to the far away galaxies.

*the thing that actually did change was that they are seen to be receeding at a faster rate than they should if the expansion were not accelerating.

Photon_2013

Bandersnatch, thanks. You are absolutely right that we can observer galaxies 9,8,7... billion LY away and draw conclusions. So according to data, about 100 galaxies closest to us show blue shift (means they are coming towards us). Suppose the last red shift galaxy we see is 1 billion LY away. Does that mean that universe was expanding 1 B years ago but it is not clear what is happening right now?

They are introducing dark enery to account for this acceleration and they seem to be coming up with precise number of 70% universe being made up of dark energy right now. However, it is not even possible to know if acceleration is still going on at outer limits of our observable universe. Light from there will reach us after X billion years, then we will know.

Bandersnatch

There's a problem with that conclusion.

We know that the expansion does not affect small-scale structures(like superclusters), and we know it from comparing our neighbourhood with galaxies far away. Even those 10bln ly away are arranged in the same kind of structures as the nearby ones.

If the movement of nearby galaxies NOW were to be explained by the effects of expansion ceasing recently, then our neighbourhood ought to look vastly different from what we see far away in space and time.

Photon_2013

It seems to me as we develop more and more sophisticated telescopes, we increase our ability to see farther in space which means we can see more in past. Eventually we will get so close to events of big bang, everything will seem bizzare and unknown. I don't think we understand the forces and energies that were present at that time, but eventually things settled down as we see now in milky way and close by galaxies. Even if some dark mysterious energy was present at that time, it may not be present now (consumed in the creation process, acceleration of galaxies etc). We are introducing concepts like dark energy just like aether was introduced at some point to account for light travelling in space similar to sound waves travelling in some medium.

Bandersnatch

It's better to have an incomplete model that produces valid predictions than not have any model at all.

Drakkith

First, we can't see beyond a certain point in time called recombination. This was the about 380,000 years after the big bang when the hot dense plasma finally cooled off enough for electrons to become bound to nuclei, allowing photons to travel freely for the first time. Prior to this the universe was opaque to EM radiation. It is not possible, not even in theory, for us to see past this point using light. So we will never reach a point where we are seeing bizarre physics we don't understand.

Second, it is already known that beyond a certain point in the past our knowledge of physics breaks down. This is precisely what many theories on Quantum Gravity are trying to fix. That point in time where we can no longer make accurate predictions is much further in the past than recombination though.

There's a huge flaw in your logic. We observe that dark energy, whatever it may be, not only existed in the past, but that it becomes increasingly important as we get closer and closer to the present. This runs completely counter to your argument.

And you're right. We are introducing concepts like the aether was introduced in the past. Why? Because that's how science works. Only through observations and experimentation can we figure out what is right and what is nonsense. The aether was a plausible idea until observations repeatedly came up with null measurements for it. After that it was discarded like it needed to be.

Photon_2013

Drakkith, please point to some specific source or literature that shows this. I did not see any evidence that DE is required to explain milky way. I thought concept of DE is only required if you try to explain all of known universe.

Thanks for comments.

cepheid

The source is General Relativity (GR). You may recall that GR says that mass-energy curves spacetime. Another way of stating this in the cosmological context: the geometry of the universe depends on its mass-energy content. In particular, the dynamics of the expansion are different depending on what constituent of the universe dominates (meaning which constituent has the largest energy density, which means energy per unit volume). In the simplest models for dark energy, it takes the form of a constant in the Einstein Field Equations (the famous "cosmological constant"). This means that its energy density is constant with time. If you think about that, it's actually kind of counterintuitive. If the energy in a given volume is constant, then as the universe expands, and the volume gets larger, the density stays the same, which means the total amount of dark energy has to increase! Only if you think of dark energy as being some sort of "vacuum energy" associated with empty space does this remotely make sense. In contrast, the energy density of matter and radiation decrease with time, since matter gets diluted with the expansion of the universe. The total amount of matter remains constant, but it gets spread out over a larger and larger volume. This is much more intuitive.

A consequence of this is that the early universe was first radiation-dominated (meaning dominated by photons and relativistic particles), and then later matter dominated (meaning dominated by non-relativistic matter). The energy density of matter and radiation was much larger than the energy density of dark energy, so matter and radiation are what contributed the most to the expansion rate of the universe. It was only later, when the matter density went below the dark energy density, that dark energy began to dominate the dynamics of the expansion, starting the acceleration that is now occurring. So the exact opposite of what you suggested is true: dark energy was relatively unimportant in the early universe and only started to become important around now-ish. This is what Drakkith alluded to.

Another member here, marcus, has a diagram in his signature that illustrates this effect. http://ned.ipac.caltech.edu/level5/M...s/figure14.jpg

The figure is a plot of scale factor vs. time. You can think of the scale factor as just a number that tells you the ratio of the separation of any two points in space at time t to their separation now. Since points were closer together in the past, the scale factor (this ratio) was < 1 for all times in the past. The evolution of scale factor with time is plotted for a bunch of different cosmological models, including ones with no dark energy (all the dashed lines). However, the cosmological model that most closely matches our observations is the solid bold line. Looking at this one, you can see that the scale factor is always increasing, (the universe is expanding), but the RATE at which it is increasing actually diminishes in the early universe. The curve starts out steep, but becomes LESS steep, showing that the expansion rate is slowing with time, just as you would expect for a standard matter-dominated universe with no dark energy. Because the gravity of all the matter in the universe tries to pull it together, it slows down the expansion. It's only once you get to t = NOW (where the grey cross is) that there is an inflection point: the curve starts to become steeper. This is due to the strange repulsive effect of dark energy, which starts to dominate the dynamics of the expansion and causes the expansion rate to increase with time.

Photon_2013

In fact, there is now an exotic inflationary proposal in which a new ingredient (a new field is introduced, referred to as ‘quintessence’, which would provide an effective cosmological constant through a dynamical ‘dark energy’ of negative pressure. It has been argued that this might be signalling a new phase of inflation coming upon us. It is to be hoped that somewhat fantastic sounding suggestions like these will indeed find rapid ways to be convincingly settled observationally, though, in practice, matters seem rarely this clear cut.

- Roger Penrose, Road to Reality (2004).

I will leave the topic of dark energy with above quote since in my mind it is too far fetched to comprehend. Those who want to believe in DE, let them believe. I would just put it in the basket - "We don't understand this".

cepheid

"Far-fetched" is not really a meaningful term to apply to physics, because what it really means is, "too far separated from my everyday experience to seem plausible to me." However, our everyday experience is largely irrelevant to our efforts to understand the physical universe. If everyday experience were sufficient, then we wouldn't need to do experiments, or construct mathematical models to describe reality, and we would already understand everything. Clearly that is not the case. The only criterion for the plausibility of a theory, at the end of the day, is its ability to match or explain our observations. Quantum mechanics and relativity both seem extremely 'far-fetched' to someone upon first being introduced to them. The reason for their sustained viability is the degree to which their predictions have been supported and confirmed by experiment.

That having been said, I certainly acknowledge that at present we have absolutely no idea what dark energy is, only what some of its properties and effects are.

Photon_2013

cepheid, by "far fetched" I meant my current understanding of physics. I was not refering to everyday experience. I know most of physics is counter intutive. But anyway, since nothing is known about DE, the discussion will not go anywhere. I would certainly hope physicists will come up with better explanation of accelerating galaxies or future experiments will reveal new critical information.

Thanks for informative posts.

Drakkith

I fail to see what's so hard to comprehend. Something is causing the universe to accelerate its expansion. We don't know what it is or how it works yet, but our observations point to this conclusion. And that's the key. Our observations lead us to this conclusion. We wouldn't even consider it for a second if our most advanced models, using data from the most advanced astronomical instruments ever invented, didn't match observations.

Photon_2013

Drakkith, everybody who works on cutting edge science uses their most advanced instruments and their best models. However they still run into wrong conclusions or reach dead ends. Happens all the time. I hope you know enough about history of science to understand that. So keep an open mind. Confidence is good for research, over confidence is not good (specially when you are working in one of the most difficult research areas). As I see it cosmology is at its beginning of beginning.