Cosmic Microwave Background – How did the CMB originate? Matter/Antimatter Annihilation and Electron Decoupling Explained

big-bangcosmic-microwave-backgroundcosmologyspace-expansion

Where did the CMB originate from? I get that at the beginning of the universe, by the Big Bang theory temperatures and pressures were too high for matter to exist, and even if it did, it would just get annihilated from its antimatter equivalent, thereby releasing electromagnetic radiation (EMR).

However, is this the origin of the CMB? From what I know, before the whole nucleosynthesis and decoupling event, the photons would constantly get scattered and hence the universe was opaque. However, after the temperatures cooled such that neutral atoms could exist, the EMR could propagate out and not get scattered.

However, I read somewhere that it was actually the decoupling event itself which was what originated the CMB. Much like flame tests, or by Bohr's second postulate, when electrons relax into the ground state or just lower energy states, EMR is released. Hence, would the relaxation of these electrons be the source of the CMB?

Best Answer

Once recombination occurred, the black-body radiation, which was in thermal equilibrium with matter (hydrogen plasma), became decoupled from it and was free to propagate. The temperature at which this occurred was about 3000 degrees kelvin.

This means that the CMB is composed of the radiation which existed right before the decoupling i.e., what was flying around inside the 3000 K hydrogen plasma right before the plasma cooled down enough to be quenched.

(Cosmic expansion then stretched out the radiation so much that its spectrum now corresponds to a very cold 2.7 degrees kelvin.)

Related Solutions

Cosmology: Is the Observable Universe Enclosed by an Infinitely Dense Shell?

When we say "the Universe" without specifying "when", we usually mean the slice of our spacetime at a constant value of $t$, the proper time since the Big Bang (at each point). This "the Universe" has no boundary or shells. Quite on the contrary, it is pretty much uniform at long enough distance scales - above 300 million light years or so.

"The Universe" that has a shell on the boundary is a different slice through the spacetime, namely the past light cone with the tip at the Earth now. Approximating the spacetime by the Minkowski space for a while, this past light cone is composed of events $(t,x,y,z)$ that satisfy $$t=-\frac{1}{c}\sqrt{x^2+y^2+z^2}$$ where $t=0$ is identified with the present. Indeed, this past light cone is surrounded by a very thick and dense wall. But you can't say that this past light cone is "the Universe" now: it is a slice through spacetime that contains some places that are photographed now (those that are nearby) and some places that are photographed in a distant past (near the Big Bang, those that are very far).

So what we see is bounded by a thick wall - and we see lots of quasars at a very high redshift (big distance, big excursion to the distant past) and we also see the cosmic microwave background - from the times when the Universe was 350,000 years or so old. We actually don't "see" before that point using photons because the Universe was not transparent at all when it was younger than 350,000 years - it was full of plasma that was absorbing, emitting, and changing the direction of photons all the time. Using other probes, we can perhaps see before the point 350,000 years.

However, it's still more accurate to say that the "thick wall" doesn't appear at distant places of the Universe at one moment; instead, it appeared everywhere in the Universe a long time ago.

Cosmology – Is the CMB Rest Frame Special and Where Does It Originate From?

I found this answer at Professor Douglas Scott's FAQ page. He researches CMB and cosmology at the University of British Columbia.

How come we can tell what motion we have with respect to the CMB? Doesn't this mean there's an absolute frame of reference?

The theory of special relativity is based on the principle that there are no preferred reference frames. In other words, the whole of Einstein's theory rests on the assumption that physics works the same irrespective of what speed and direction you have. So the fact that there is a frame of reference in which there is no motion through the CMB would appear to violate special relativity!

However, the crucial assumption of Einstein's theory is not that there are no special frames, but that there are no special frames where the laws of physics are different. There clearly is a frame where the CMB is at rest, and so this is, in some sense, the rest frame of the Universe. But for doing any physics experiment, any other frame is as good as this one. So the only difference is that in the CMB rest frame you measure no velocity with respect to the CMB photons, but that does not imply any fundamental difference in the laws of physics.

“Where does it come from?” is also answered:

Where did the photons actually come from?

A very good question. We believe that the very early Universe was very hot and dense. At an early enough time it was so hot, ie there was so much energy around, that pairs of particles and anti-particles were continually being created and annihilated again. This annihilation makes pure energy, which means particles of light - photons. As the Universe expanded and the temperature fell the particles and anti-particles (quarks and the like) annihilated each other for the last time, and the energies were low enough that they couldn't be recreated again. For some reason (that still isn't well understood) the early Universe had about one part in a billion more particles than anti-particles. So when all the anti-particles had annihilated all the particles, that left about a billion photons for every particle of matter. And that's the way the Universe is today!

So the photons that we observe in the cosmic microwave background were created in the first minute or so of the history of the Universe. Subsequently they cooled along with the expansion of the Universe, and eventually they can be observed today with a temperature of about 2.73 Kelvin.

EDIT:

@starwed points out in the comments that there may be some confusion as to whether someone in the rest frame is stationary with respect to the photons in the rest frame. I found a couple more questions on Professor Scott's excellent email FAQ page to clarify the concept.

In your answer to the "How come we can tell what motion we have with respect to the CMB?" question, there is one more point that could be mentioned. In an expanding universe, two distant objects that are each at rest with respect to the CMB will typically be in motion relative to each other, right?

The expansion of the Universe is certainly an inconvenience when it comes to thinking of simple pictures of how things work cosmologically! Normally we get around this by imagining a set of observers who are all expanding from each other uniformly, i.e. they have no "peculiar motions", only the "Hubble expansion" (which is directly related to their distance apart). These observers then define an expanding reference frame. There are many different such frames, all moving with some constant speed relative to each other. But one of them can be picked out explicitly as the one with no CMB dipole pattern on the sky. And that's the absolute (expanding) rest frame!

Assumptions: From most points in the universe, one will measure a CMBR dipole. Thus, one would have to accelerate to attain a frame of reference "at rest" relative to the CMBR. Questions: Having attained that "rest frame", would one not have to accelerate constantly to stay at rest (to counter attraction of all the mass scattered around the universe)? [abridged]

I think the assumption is wrong, and therefore the question doesn't need to be asked.

The fact that there's a CMB dipole (one side of the sky hotter and the other side colder than the average) tells us that we are moving at a certain speed in a certain direction with respect to the "preferred" reference frame (i.e. the one in which there is no observed dipole). To get ourselves into this dipole-free frame we just have to move with a velocity which cancels out the dipole-producing velocity. There's no need to accelerate (accept the rapid acceleration you'd need to do to change velocity of course).

Our local motion (which makes us move relative to the "CMB frame" and hence gives us a dipole to observe) is caused by nearby clusters and superclusters of galaxies pulling us around. It's true that over cosmological timescales these objects are also moving. And so if we wanted to keep ourselves always in the dipole-free frame we'd have to make small adjustments to our velocity as we moved and got pulled around by different objects. But these changes would be on roughly billion year timescales. And so to get into the frame with no CMB dipole basically just requires the following 3 steps: (1) observe today's dipole; (2) move towards the coldest direction at just the right speed to cancel the dipole; and (3) maintain basically that same velocity forever.

Best Answer

Related Solutions

Cosmology: Is the Observable Universe Enclosed by an Infinitely Dense Shell?

Cosmology – Is the CMB Rest Frame Special and Where Does It Originate From?

How come we can tell what motion we have with respect to the CMB? Doesn't this mean there's an absolute frame of reference?

Where did the photons actually come from?

EDIT:

In your answer to the "How come we can tell what motion we have with respect to the CMB?" question, there is one more point that could be mentioned. In an expanding universe, two distant objects that are each at rest with respect to the CMB will typically be in motion relative to each other, right?

Related Question