Hi Courtney. I understand that the hotspots in cosmic background radiation are said to be where matter first came together. Ive always been a little confused by this. I gather there are ways we filter out other radiation to leave just the cmbr? Presumably other radiation comes from matter (suns etc). What I find confusing is that we are looking at hotspots that lead to conglomerates of matter that then give off radiation which we attempt to filter off. In short, isnt there now matter where we find the hotspots of cmbr? We find a hotspot in a certain direction of the sky-should there not be a galaxy right there now?...and if there was this would obviously interfere with getting an accurate cmbr reading! Hope i am making at least some sense! Also, on a similar note.  The cmbr is aprox 3 kelvin? Am I alone in thinking that: this is exactly what I would have guessed it to be with absolutetely no evidence! My reasoning,,space I wouldve guessed to be zero degrees and the sparse suns and other bodys wouldve slightly warmed it up. If in the past space was zero degrees but the suns etc were closer together they would have less space to heat up, so the cmbr would be warmer...which is what we think it would have been. Am i simply stating the obvious, or is my understanding of the cmbr all wrong. I would appreciate any comments you could make.

(Sorry for the delay. I was busy yesterday afternoon and evening, and didn't receive your note until this morning.)

(1) The hot spots are not where there was more matter; they are where there was less matter. Here is the reasoning involved:

The CMBR represents the furthest and therefore earliest moment that we can see. During and for a while after the Big Bang the Universe was so dense that it was opaque, meaning that as in the inside of a star, any light heading in any direction has too much stuff in the way to have any hope of getting out. Instead, it bounces from pillar to post, so to speak, slowly making its way from one place to another in a series of relatively small steps.

However, during this time the Universe was rapidly expanding, which means its density was rapidly decreasing. At some point it had a low enough density that light heading in some direction might just keep going, more or less forever, because there wasn't enough stuff in the way for it to have much chance of running into anything, and during the time it took to get to where something might have been, the continuing expansion moved it out of the way. This is the moment in time that we see as the CMBR.

However, the moment that we as the CMBR in one place may not be the same as at another place. If there are places that are denser, they have to expand more to have low enough density for light to escape. If there are places that are less dense, they don't have to expand as much. The result is that denser places are seen at a later date, when the Universe had expanded more, and less dense places are seen at an earlier date, when the Universe had expanded less. Now suppose that at a given time every place in the whole Universe has about the same temperature, and that as it expands the expansion of the gas in the Universe is gradually cooling it. If you look at a place that has more matter and is therefore denser, and therefore has to expand more to become more nearly transparent, it will be cooler when you can finally see it. If you look at a place that has less matter and is therefore less dense, and therefore does not have to expand as much to become more nearly transparent, it will be hotter when you finally see it.

In other words, the hot spots in the CMBR are not where there was more stuff, they are where there was less stuff. The cool spots are where there was more stuff. I know it seems backward, but that's the way it is/was.

(2) What you see "at" the CMBR is not necessarily what you see "in front of" the CMBR.

When you look at the CMBR you are seeing the various regions that are so far away that we are just now seeing the light that left them at the end of the "Cosmic Fireball". When you look at things "in front of" the CMBR you are not looking at the same regions later on, you are looking at completely different regions that happen to be closer to us, so that we see them as they were later, because the light they emitted at the end of the Cosmic Fireball has already passed us. Just as there are differences in density between different places that we still see as the CMBR, there are differences in density all along the line of sight between here and there. So although there might be a cold spot in the CMBR, meaning more material happened to be in that place at that time, things "in front of" that, since they are different regions, could have the same mass, or more or less mass.

This means that what we see in front of the CMBR is as random as what we see in the CMBR itself. There might be as much or more material in front of a cooler spot in the CMBR as "at" that spot, or somewhat less material. So there is not likely to be any correlation between what we see in front of the CMBR and the CMBR's appearance. All that we can expect to see is that at different distances along the line of sight there will be random clumps of matter and random voids, none of which have any relationship to each other or their background.

(3) The temperature of the CMBR, as we see it, is the temperature that the gas in that part of the Univers had at the time it became more or less transparent divided by z , the redshift associated with that part of the Universe as we now see it. The last I recall reading about this, the actual temperature "at" the CMBR was of the order of a hundred million Kelvins, but the redshift at the distance involved was of the order of 30 million, giving a ratio of 100 million divided by 30 million or about 3 Kelvins as the apparent temperature of the microwave background. I don't recall exactly how the number was first estimated, but I'm certain that you have to have some idea of what conditions might have been like at that time to do the calculation. If you didn't have to know anything about the conditions, then the number would have to be insensitive to them; but as you know there are brighter hotter spots and fainter cooler spots, which correspond to less dense and more dense regions. So if the Universe had more mass the CMBR would be fainter and cooler, and if it had less mass it would be brighter and hotter. As I noted above it has been a very long time since I read about this in any detail, but I believe a simplifying assumption was made that the mass of the Universe was near the "critical mass" (or more accurately, density), at which it would never stop expanding, but would almost stop, given enough time. As it happens the actual mass is about a quarter of the critical mass (counting dark matter, whatever that is), which is not "off" by that much. But at least theoretically, the mass could have been much, much less or much, much more than it is. So the temperature of the CMBR could have been higher or lower than it is. Of course, since it is so low, even a considerable relative change in the value wouldn't amount to more than a few degrees, so it would have to be somewhere near the value we observe; but the specific number is almost certainly not something that can be calculated without any knowledge of the nature of the Universe.

So no, I do not believe that you could figure it out without referring to the facts.


All Answers

Answers by Expert:

Ask Experts


Courtney Seligman


I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.


I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

©2017 All rights reserved.