# The Cosmic Microwave Background? Not always…

The Universe isn’t a static place. Although the laws of nature (physics, chemistry, biology, etc.) don’t appear to change over time, everything in the Universe appears to evolve, and changes over time. One of the simplest ways that this happens is through Hubble expansion.

General relativity tells us how the Universe expands, and more specifically, it tells us that the expansion rate (which we call the Hubble constant, H0) is related to the total energy density of the Universe. More matter density: faster expansion. Greater density of photons (i.e., light): faster expansion. More of any and all types of energy (even the mysterious dark energy): the expansion rate is faster.

The weird thing is, it isn’t just that the density of stuff causes the expansion rate to be what it is. Because the Universe is expanding, the energy density of everything in it is changing. If we know what the energy density of all the stuff in the Universe is at any one moment, we can reconstruct the entire expansion history of the Universe, from the big bang until as far into the future as we like:

We can also figure out how things like the matter density changed over time. As space expands, the volume of the universe increases, and we know that matter density dilutes inversely proportionate to volume. That is, as the volume goes up by a factor of eight, the matter density becomes one-eighth of what it previously was. But photons, also known as light or radiation, do something extra. Matter, remember, is just ordinary junk, like my significant other’s car’s transmission. But light is a wave, and when the Universe expands, the light’s wavelength expands with it, like so:

Now, when light has different wavelengths, it has different properties, including different Energies. Gamma rays are the most energetic and have the shortest wavelengths; radio waves are the least energetic and have the longest wavelengths. (In fact, radio waves have so little energy that when you jump into a swimming pool, that expends more energy than is present in all the radio waves ever received on Earth!) Energy is also inversely related to wavelength:

So now, let’s combine these two things we know about the expansion of the Universe: as the Universe expands the Volume of it increases, and the wavelengths of all the light in it also increase. This means that the Universe was hotter in the past, and will be colder in the future. This means that if the volume of the Universe increases by a factor of eight, the energy density of photons is one-sixteenth of its initial value, not one-eighth like matter.

At some point in the past, the Universe was so hot that neutral atoms couldn’t form! Whenever a proton and and electron combined to form neutral hydrogen, a photon would run into it, and would be so energetic that it would knock the electron out of the atom. The Universe was so hot that it did this continuously for the first 380,000 years of its existence. But because it expanded and cooled, eventually the photons lost so much energy that they couldn’t keep all the atoms ionized, and they became neutral.

When the atoms became neutral, the photons became free to travel through space without running into unbound protons and electrons. This leftover radiation from the early universe is what we see as the cosmic microwave background today.

But why is it a microwave background? Well, when the Universe first became neutral, most of those photons were only a little bit too red to be visible, and there were a bunch that were visible light, and even some that were ultraviolet light (those are the ones that kept the Universe ionized for so long). But the Universe has expanded by a factor of over 1,000 on each side since then, meaning that every single one of those photons is a factor of over 1,000 less energetic. That means the photons which used to be ultraviolet are now near- and mid-infrared, the ones which used to be visible are now far-infrared, and it means that the radiation which was once infrared (which was most of them) are now in the microwave. Hence, today we have a cosmic microwave background. It also means that, billions of years from now, as the Universe continues to expand, the microwave background will continue to shift, eventually becoming a cosmic radio background. How neat is that?

Also, if you have the time and interest, the latest Carnival of Space is up, and Starts With a Bang! has two links on it this week. Enjoy, and thanks for reading!

### 11 thoughts on “The Cosmic Microwave Background? Not always…”

1. Ethan, Starts with a Bang rocks! The layout and visuals are really attention-grabbing. And the content is fascinating, (well to me anyway and to the few lost souls who are interested in cosmology). I particularly like the graphics and diagrams which help to explain complex concepts in a simple and easy-to-digest way. Personally, I believe the universe is alive, aware, intelligent and growing. Every night before I fall asleep I recite the following mantra: “It’s emergent, it’s dynamic, it’s a process–but what is the substrate?” Anyway, I’ve rambled on too much in this commment. I have listed Starts with a Bang on the Cosmic Rapture blogroll. It would be gfreat to see Rapture listed on the Starts with a Bang blogroll. Cheers, SRS

2. Ethan, First, let me echo the kudos offered by Steven Schwartz, a terrific job. Well done!
My question for you regards your phrasing, “the Universe has expanded by a factor of over 1,000 on each side”. I believe the universe is finite and bounded, and I have trouble understanding what is meant by the phrase, “the size of the universe doubled”. Is it the volume that has doubled, or, if you think of the universe as a spheroid, is it the radius that has doubled? Your use of the phrase “on each side” has prompted my question. Can you clarify? Thanks.

Frank

3. Hi Steven and Frank,

Thanks for the compliments! I will keep updating this site and trying to make it better! As far as we know, the universe is not necessarily either finite or bounded. What is finite is how much of the universe is causally connected to us, or how big a volume could light that was emitted at the big bang have filled by now. As the universe gets older, light travels farther, and that’s a good conceptual way to understand how our universe, or the parts of it that we can see, got bigger. If you’re thinking of it as a spheroid, then yes, it’s the radius that goes up by a factor of 1,000, and the volume therefore goes up by a factor of a billion!

4. Hi guys. Nice site and explanations. I have a couple of questions. Sorry if theyre dumb, but Im not a scientist.

Firstly, as I understand it, when we look through a telescope we see light from stars/galaxies/whatever thats taken X light years to reach us. The farthest we can see are objects around 15 billion light years away, i.e. roughly the age of the universe.

Now, if 1) that light has taken 15 billion years to get to me, and 2) 15 bilion years is the approx age of the universe, and 3) Im looking at the object as it was 15 billion years ago, not as it is now, why do I see a galaxy and not a “singularity”? I know we cant see a singularity cos the universe was opaque for 300,000 years, but do you see my point? If Im looking at something, the light from which has taken 15 billion years to get to me, and there was only an opaque ball of radiation and stuff 15 billion years ago, why do I see formed galaxies? Shouldnt the age of the universe be: TIME LIGHT FROM OBJECT TAKES TO REACH ME + TIME TAKEN TO FORM OBJECT IM LOOKING AT?

Secondly, when we look at CBR, are we seeing microwaves that have been travelling TOWARDS us since the moment the universe stopped being opaque, or are we picking up microwaves that are bouncing around (and have been bouncing around since the universe stopped being opaque) the same “chunk” of space that we now inhabit, i.e. they really are BACKGROUND and have simply expanded along with us as the universe expanded?

Im confused because the universe was much smaller when these CBRs were released than it is now. When I look outwards at space, Im the centre of a 15 billion light year sphere… yet in every direction I see CBR… if that CBR is coming AT me, from ALL directions, I cant see how the universe could have been smaller in the past… Surely if when I look out at the universe, and effectively pick up information about the past, Id have to know which direction to look to see the baby universe?

I have read that gravity acts as a lens…. does that mean that looking at the CBR (thats been travelling for 15 billion years) in WHATEVER direction, my line of sight, in ANY direction, is actually focused back to the same point?

5. Andy,

The cosmic microwave background is (much more than 99%) the stuff that’s been traveling towards us since the big bang — very little is stuff that’s been bouncing around. I think to adequately answer your comment will require a long post on its own, but it may take me a week or so to get the requisite stuff together. If you can distill your question down to the single point you want to know, most importantly, I can take a good crack at it.

Best,
Ethan

6. OK. Thanks. The more I think about it the more confused I get, and the more questions appear. I suspect my trouble is cos the universe isnt a 3D sphere like Im imagining.

What Im having trouble understanding is how the microwave radiation arrives here after travelling for 13.4 billion years in a straight line, since the moment of last scattering.

I dont know the “radius” of the universe at that point, but lets assume it was 300k light years. That would give it a surface area of 4pi R^2 = A. (Youll probably tell me here that the universe doesnt have a surface area! 🙂

As I understand, the universe stopped being opaque 13.4 billion years ago, and the CBR set out on its trip, some of it finally reaching earth today from every conceivable direction. i.e. its coming from the edges of the earths light sphere, which has a radius of 13.4 billion years, and a surface area 2×10^9A …

See my problem? The “area” the CBR emanates from is much larger than the one it set out from…

My next problem (and I suppose I need to do a doctorate in cosmology to even begin to understand) is how the expanding universe affected this CBR.

If we imagine the universe at the moment of last scattering to be that famous balloon Ive seen people use to explain the expansion of the universe (I know its a 2D surface is not the same as the universe, although I have no idea what this actually means or implies) and we mark two separate points, one representing a photon that will travel in a straight line and arrive at an observer 13.4 billion years in the future, and the other the “stuff” the observer ends up being made out of.

When we press “go!”, the balloon gets blown up and the photon heads off in the direction of the observer at the speed of light.

Now what happens?
Where would the point that represents the photon be marked on the balloon relative the observer?
No matter what the original separation, these points cant be 13.4 billion light years apart. The maximum separation would be the diameter of the balloon before it got blown up.

So how does the expansion of the universe affect the speed of light? Like an athlete running at a constant 20mph on an elastic track towards the finish, which keeps getting stretched away from him?

Thanks.

7. Thanks. Wont pretend I understand the expansion-faster-than-light idea, but you cleared up the distance thing.

Refering back to my other question, why does the CBR “appear” to come from a light sphere that “appears” NOW to be larger than the universe WAS when it first set off in a straight line on its 13.4 billion year trip??? Any comments?

8. Wow. You know, you asked that, and my brain went, “Oh, that’s just what’s known as ‘the homogeneity problem’ in cosmology!” I know the answer to that one, but I figured there must be a good explanation out there. Then I googled the homogeneity problem and realized, HOLY CRAP! There’s nothing out there explaining that in a comprehensible way at all! Stay tuned — thanks for another great question!

9. IS UNIVERSE THE MATTER CONTAINED IN SPACE OR UNIVERSE IS SPACE CONTAINING MATTER? IF UNIVERSE IS BOTH THEN IT CAN NOT EXPAND OR CONTRACT. (EXPAND TO WHERE IF THERE IS NO SPACE BEYOND). THEN WHAT IS BEYOND SPACE THAT CONTAINS THE UNIVERSE? INTERESTING THAT VACUUM CONTAINS NOTHING (NO MATTER, NO ENERGY)HOWEVER, IT CONTAINS ALL MATTER/PARTICLES). IF YOU SAY GOD IS NOTHING (DOES NOT EXISTS) THEN I AGREE WITH YOU TO MAKE MY POINT THAT VACUUM IS ALSO NOTHING (DOES NOT EXIST). BUT I FEEL FOR SURE THAT IF THERE IS NO VACUUM THEN WE DO NOT HAVE A UNIVERSE. BUT WE HAVE UNIVERSE SO WE MUST HAVE WHAT? THAT IS MY CONFUSION IF YOU CAN RESOLVE.

10. Tom, our current theory is that space itself is expanding, and that our Universe, although very large, may or may not be infinite.