Let's Talk Physics

A stochasticly updated blog about interesting topics in Physics & Astronomy

Dark Energy, and the return of the Cosmological Constant

Dark Energy is a very hot topic in physics at the moment, and has been for the last decade or so. So far, not a lot is known about what dark energy is, or where it comes from. We have a few ideas, but nothing solid yet. Before I talk about these ideas, I’m going to talk about why we think it exists.

As previously mentioned, it’s very hard to talk about modern ideas in physics and not end up back with Einstein at some point, and this story is no exception. Actually, this story starts with Einstein. 2 blog posts ago, I wrote about Einstein’s “greatest blunder”, as he called it. To refresh your memory, here’s what his blunder was in a few key points

  • Einstein derived his famous field equations, and found that his equations predicted the Universe should be expanding, which no one believed at the time.
  • Einstein added a constant to his equations, called the cosmological constant. His equations now stated that the Universe was neither expanding or shrinking.
  • A few years later, Hubble confirms Lemaître’s theory that the Universe is, in fact, expanding.
  • Einstein removes the cosmological constant, calling its addition the “greatest blunder of his life”.

This all happened in 1929. Einstein’s field equations went untouched for the next 69 years. Then, in 1998, Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess discovered that the Universe’s expansion is accelerating. To make Einstein’s equations fit with how the Universe was behaving, it had to be altered again. And guess how? That’s right, scientists reintroduced the cosmological constant (albeit, with a COMPLETELY different value). The cosmological constant appears in Einstein’s field equations as a Λ.

The cosmological constant returns

The cosmological constant returns

So, after arriving at this amazing conclusion that the expansion of the Universe was accelerating, physicists were left the incredibly difficult problem of figuring out what was causing this acceleration? Previously, when people knew that the Universe was expanding but didn’t know about this acceleration, everyone assumed that the expansion would eventually stop as gravity would slow the expansion down and eventually start pulling everything together. Now, scientists had to figure out what was combating gravity – and what they came up with is dark energy.

Everything we can see in the Universe is composed of matter. For a long time, people assumed that this was everything in the Universe, and that there is a lot of empty space in the Universe – all you need to do is look up to see the amount of darkness compared with light to figure this out. But then, astrophysicists began looking at the structure of other galaxies. And what they found is that the galaxies they were observing should have a lot, lot more mass than we can see for them to maintain their structure. Which led to the idea of dark matter, which is exactly what it says on the tin – it’s matter that emits no light. It is completely invisible. Thankfully, we have another way of confirming if it exists. If you have a body with a lot of matter (like a blackhole, or a body of dark matter) then the body can bend light that passes close to it. This is known as gravitational lensing, and this lensing due to dark matter objects has been observed as these dark matter objects pass in between us and other galaxies.

The ring in the middle right section of the image is a galaxy whose image has been warped into an Einstein ring by Gravitational Lensing

The ring in the middle right section of the image is a galaxy whose image has been warped into an Einstein ring by Gravitational Lensing

Right now, scientists think that there’s approximately 4 times more dark matter in the Universe than there is normal matter. But wait a minute – this dark matter is only going to increase the strength of gravity, slowing the expansion of the Universe quite a lot. So if it’s actually accelerating, then this means that there is something powerful out there that’s overcome the combined gravity of matter and dark matter. And that thing is, we think, dark energy.

How it works is a bit of a mystery. What we currently think is that dark energy is everywhere in the Universe, but virtually undetectable by normal experiments. And since dark energy if everywhere, it must take up a fraction of the energy in the Universe – after all, matter is energy, as is dark matter, and there is a finite amount of energy in the Universe. Current measurements, taking into account the total amount of matter and dark matter in the Universe and using this along with the rate of acceleration of expansion of the Universe, show that the composure of the Universe is

  • 4% normal matter
  • 23% dark matter
  • 73% dark energy

That is quite a lot of something we can’t detect!

There are 2 popular ideas of what dark energy is. Firstly, it is a property of space itself. That is, empty space has an energy associated with it. This “energy-of-space” (as described in this article on the NASA site, and from which I’m paraphrasing quite a lot) forces space to expand slightly. And now that there is more space, there is more energy again, since the energy is associated with the space itself. So this causes the space to expand a bit more, a bit faster, and leads to our acceleration that we’ve been looking for. But, this idea is still not fully understood (or at all, really.)

To try and explain this a bit better (and with some calming music in the background) I’m going to hand you over to the Minute Physics youtube channel, to make sure we’re all on the same page.

Another idea for what dark energy might be is as follows – the vaccum of space is full of particles that pop into and out of existence. Sounds kind of made up- but it’s a phenomenon that has been observed before. So when scientists tried to figure out how much energy would be contained within the vaccum if it had these virtual particles, the answer was 10^120 (that’s 10 followed by 120 zeroes) bigger than what our model predicts.

So what’s going on? Honestly – we have no idea. None. This is an open research problem that physicists are crying out for people to try and solve.

————————————

I’m not going to lie, I found this article really hard to do. Dark Energy is just that strange of a phenomenon, and there is just too little about it known (actually known, not just guessed) to give you a real feel for what it is. But I hope you enjoyed it anyway! So send in your suggestions for Monday’s article. Also, if you haven’t, sign up to the mailing list on the right of the article. See you on Monday!

Mark

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About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

One comment on “Dark Energy, and the return of the Cosmological Constant

  1. Rick Ryals
    March 5, 2013

    I’m not going to lie, I found this article really hard to do. Dark Energy is just that strange of a phenomenon, and there is just too little about it known (actually known, not just guessed) to give you a real feel for what it is.

    What I find ironic about this is the way that people say in one breath that Einstein’s cc has been given a reprieve, while claiming that we don’t know what it is. Had Einstein’s cc actually been brought back for serious consideration, then we’d know that dark energy is the same mass-energy that comprises ordinary matter, but it is less dense than ordinary matter, so it has negative pressure.

    The gravitation effect of matter was exactly cancelled by the cc and he had a total density of 1.5*rho(matter), with a total pressure of -0.5*rho(matter)*c^2.

    The vacuum has negative pressure when P=-u=-rho*c^2. Pressure is proportional to -rho, but pressure is negative in an expanding universe so energy density is still positive. It is essentially ordinary mass-energy that is mimicking a negative mass object by way of its anti-gravitational effect.

    The vacuum energy density is less than the matter energy density, but it is still positive, and positive matter density can be obtained locally if you condense energy from this negative pressure vacuum into a finite region of space, until the energy density over this region equals that of the matter density. This will, in-turn, cause negative pressure to increase, via the rarefaction of Einstein’s vacuum energy, so this expanding universe does not run-away, because the increase in rho is offset by the increase in negative pressure that results when you make particles from Einstein’s vacuum energy.

    In Einstein’s model, G=0 when there is no matter. The cosmological constant came about because we do have matter, so in order to get rho>0 out of Einstein’s matter-less model, you have to condense the matter density from the existing structure, and in doing so the pressure of the vacuum necessarily becomes less than zero, P<0.

    The off-set increase in both mass-energy and negative pressure that matter generation entails indicates that an expanding universe is not unstable, nor will it "run-away", because Dr. Einstein's equation, g=(4pi/3)G(rho(matter)-2rho(vacuum))R=0

    Einstein didn't know about the particle potential of the quantum vacuum or maybe we wouldn't be having this conversation today.

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This entry was posted on February 28, 2013 by in Astrophysics and tagged , , , , , , , , , , , , .
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