I get email: and then assign it to my students

I’m teaching an advanced undergraduate/graduate course in cosmology this term, and I thought it might be fun to combine the education and outreach sides of my life. I get sent a lot of interesting questions, and answering them have prompted me to learn about subjects that I might not otherwise have thought much about, as well as improving my ability to focus on the mindset of my readers. I figured I’d give my students the same opportunity for a little extra credit.

I picked a few cosmology oriented questions and asked my students to take a swing at them. I’ll post some over the next few days, beginning with this question about the nature of the expanding universe.

Dear Physicist,

Ok, I know about how astronomers have determined the Universe is expanding but there seems still to be a question of why. I thought of a possible reason but surely a non-physicist as myself is not aware of all the possible reasons already rejected or factors already calculated in explaining the observations. Here is a thought that has me plagued. Please tell me what may be wrong or right about it.

The gravitational force between bodies decreases with distance. That much I know. Maybe the separation of mass in the universe decreases the gravitational forces between bodies at a rate faster than the decrease of stored kinetic energy in those bodies? Could it then be that the combined result of force vectors is causing distance between those bodies to accelerate? Also, perhaps as objects move out into space with less matter, solar winds etc, that the resistance of space decreases with distance from the big bang. That may also cause a further acceleration of expansion from reduced drag. Could those two factors explain the observed expansion of the Universe?

My talent in math and physics is not good enough to figure this out but I would guess yours is. I hope you get back to me and end my sleepless nights.



From Anna:

Hi Paul,
You’ve asked an important question – why is the universe expanding? – a concept that even Einstein himself had trouble believing. In 1929, Edwin Hubble (whose name you may recognize from the Hubble Telescope) famously demonstrated that galaxies appear to be receding from us at a speed proportional to their distance from us. The relationship is speed = H*distance, where H is now called Hubble’s constant. Upon hearing this, you might think that if every galaxy we observe is moving away from us, then doesn’t that imply that we are at the center of the universe? There is a way out of this of course, if the entire universe is expanding – think stretching out a rubber sheet in al directions – then every point in the universe is moving away from every other point. Hubble’s observations have been repeated numerous times, and we now have a fairly accurate estimate of the H value. So there’s no getting around it – the universe is expanding.

But the perplexing question is this: given that we know the universe is made of matter (stars, galaxies, gas), and that matter gets attracted together by gravity, why is the universe expanding and not collapsing on itself? This is precisely the reason Einstein did not like the idea of an expanding universe. In the 1920’s, Einstein’s general relativity equations were being put to use in describing the geometry of the universe (no easy task). Einstein worked with the physicist Willem de Sitter and each developed models of the universe that assumed (as any reasonable person might) that the only “stuff” in the universe is matter. However, both of these models yielded a universe that is unstable. Einstein realized that the model could be made stable if an extra term was added in, what he called the “cosmological constant.” The constant allows for a universe that does not collapse in on itself, but rather expands. The idea greatly disturbed him, because he was hoping that this equations would yield a static universe that was basically unchanged throughout time. In 1947 he said: “Since I introduced this term, I had always a bad conscience… I am unable to believe that such an ugly thing should be realized in nature.” However, today the cosmological constant is accepted in the standard theory of cosmology, and has been measured to have a value of about 0.7.

Sparing you too many details of the physics, the cosmological constant – this factor that allows for expansion in the face of gravity – arises from what you may have heard called “vacuum energy”. There is a result from Quantum Mechanics that says even empty space has a small but non zero energy, a very strange discovery indeed! One consequence of this, is that this small amount of energy has “negative pressure”, yet another strange concept. All this means is if pressure is to “push down on something”, then negative pressure simply means to push out or expand. So this vacuum energy, or more properly we call it dark energy, is what works to expand space itself.
I hope this serves as a satisfactory answer to your question. If it does, and you would like more to think about, I’ll tell you that not only is the universe expanding, but it is expanding at an ever faster rate. Three physicists proved this in 1998, and were just this year awarded the Nobel Prize. If you are in the area, one of them will be giving a talk at Drexel called “The Accelerating Universe” on March 1st. This talk is held every year by the physics department at Drexel and is meant for physicists and non physicists alike. More information can be found here.

From Justin:

Dear Paul,

The current explanation that physicists have concerning the expansion of the universe is something we like to call ‘dark energy’. Mathematically, this is similar to the ‘cosmological constant’ in Einstein’s abandoned addition to his general theory of relativity. As to what this stuff physically is, however, we really don’t know.

We do know that the universe is indeed expanding, from the measurement of the Hubble constant at far distances. It has been only relatively recently that this has been ascertained; in 1998 the two teams of Saul Perlmutter, and Brian P. Schmidt and Adam G. Riess measured the most distant supernova distance and determined the expansion as the result (the accomplishment earning them the 2011 Nobel Prize in Physics). The Hubble constant relates the recession velocity of a particular object with its distance from us. Found by observing the redshift of objects (the lengthening of a known signal’s wavelength by expansion), we know that each object we observe is moving faster away from us the farther it is away from us. To think about this in an analogy, you can think of a rubber sheet. If you can imagine certain marked points on this sheet, when one stretches this sheet out in all directions, these points will spread out from each other, without actually moving on the sheet. In our universe, these marks can be star clusters, galaxies, etc. While each individual component may have its own random velocities compared to other components, in general the entire system is expanding.

You are correct in the reduction of the gravitational force according to distance. However, this gravitational attraction is insufficient to explain the total expansion of the universe. If you can imagine the farthest bodies observable, (let’s say a distant galaxy or quasar) then these may be subject to very small gravitational forces towards the center of the universe, and may very well be moving without any real ‘drag force’ towards the farthest regions of space we know of. However, we also know that the entire universe is expanding, not just those objects extremely far away from us. Knowing the speeds that galaxies and stars are capable of obtaining from other areas of astronomy, we find that the expansion speed far exceeds these characteristic velocities in some cases, and that, among other statistical factors, it is space itself that must be expanding.

This is where the concept of dark energy enters into play. As cosmologists, we must account for what kind of ‘stuff’ comprises our universe. Without getting into mathematical detail, we find these components: matter (both the kind we can see and the kind we can’t – the supposed dark matter), relativistic particles (including radiation and neutrinos, etc.), the inherent curvature of space (do we live on a 3-dimensional globe, or flat space, etc.), and the ‘cosmological constant’ that Einstein predicted (dark energy). This last term is another, completely different, variable in our equations of the universe. Without this, we could not produce a model that is consistent with all the things we know about our universe, from the very beginning straight up to the present day. An important point is that this factor does not have the same behavior as the variables for matter, radiation, and space curvature do. That is, it is something completely unrelated to all of these things that we presently know of, and furthermore, it seems as if it is the largest component of the universe by far. Our current model of physics does not predict what this ‘dark energy’ could be, only that either something like it exists, or that some aspect of established physics must be ‘broken’. As to the form of this dark energy, some theories have been proposed, such as a constant energy density pervading all space, yet remaining undetected by us, or of the existence of a new fundamental force (quintessence), but so far these remain solely postulates with no experimental evidence.

And so we are left with a wide-open definition of ‘dark energy’ explaining the accelerating expansion of the universe, but have no direct evidence of what it really is, only ruling out that it cannot be composed of the normal components of the universe that we already know of.


Finally, from Karsten:


Hello! My name is Karsten and I am a first year graduate student in Dr. Goldberg’s cosmology class. I am going to take a stab at your question and attempt to shed some light on the dynamics of the universe! It is my sincere hope that when you finish my email, this problem will no longer plague you but simply bother you to the point of sleeplessness.

From what I read, you have a solid understanding of the Big Crunch theory – the idea that following the Big Bang, the gravitational attraction of matter will inevitably cause everything to stop expanding and then recompress into a tiny ball of universe. Honestly, that was also the theory that I subscribed to before taking Dr. Goldberg’s class. Our universe, it seems, will inevitably end in a Big Chill, where the universe’s expansion accelerates and stars are no longer able to form.

Why is this, you ask? You are correct that the gravitational force does decrease with distance, but never does the gravitational force between two objects ever go to zero. This means that if you have a universe where gravity is the only thing that’s holding it together, then no matter how large it expands, it will slow down and it will crunch back up. It will just take a really, really long time. This suggests to us that something is, in essence, acting against gravity, at least when it comes to the expansion of the universe. Additionally, I can address your theory that the decreased density of the universe as a result of it getting bigger could play a part in its expansion. Intuitively, I would agree with you that there is less stuff for matter to bump into so it would tend to continue its outward expansion…but, to me at least, that would only lengthen the inevitable Big Crunch and would not explain why the universe is expanding at faster and faster rates!

You have probably heard the terms “Dark Energy” and “Dark Matter” being thrown around. Well, Dark Energy is what cosmologists name as the culprit in the case of universal expansion. Why is it called Dark? Well, that seems to be because we can’t directly measure it and don’t know what it is! Dark Energy is a place holder for whatever energy it is that is causing the universe to accelerate, kind of like how the ancients didn’t understand why lightning struck the Earth and came up with the idea of a god hurling lightning bolts at us. A number of scientific experiments have suggested to us the presence of this Dark Energy, here are a few.

First of all, you might have heard of red or blue shifting. It’s a phenomenon that occurs when cosmic bodies that emit light are moving toward or away from us. When they move toward us, the light they emit (when viewed right up next to the thing) looks bluer to us if its moving toward us and redder if it is moving away. By observing many supernovae of the Type IA variety, we were able to see that almost everything around us is moving away from us! From this data we were able to conclude that the universe is expanding and that expansion is accelerating.

So, to get back to your question about what makes the universe behave as it does, I can tell you that there are four things that contribute to the expansion of the universe as we know it. If you imagine the universe at the very beginning like a big explosion, then you’ll have no trouble in believing me when I tell you that radiation plays a part in the expansion of the universe. Matter affects universal expansion as you well know and now you know that Dark Energy is the fourth component. If you were to keep track of which of these four things (radiation, curvature, matter or dark energy) were important and when, then the timeline would go radiation, curvature, matter, and dark energy. In fact, dark energy is now the dominant factor for why the universe does what it does and the gravitational attraction of visible matter will play less and less a role as time goes on.

In closing, I hope I was able to answer your question and not ramble on too much. Feel free to email me back if you’d like to chat some more about the universe. I don’t know much but I find it a fascinating topic of discussion.


As always, I’m happy to get your email, but if you have any followups for the expert panel, feel free to leave them in the comments.


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