Expert: Courtney Seligman - 11/10/2010

Question
Hi Courtney. I have a number of points that i would like your comments on please. Dark matter has invoked various theories on its nature, some seem rather complicated. If we follow occams razor, would not a simpler explanation be that the matter we can observe is more massive than we realise?!! Has anyone tried putting in different figures for the mass we do see, to see if that gives the galactic motions we see?  Could you briefly explain to me why the CMB must be from the big bang. Its just that each time i read about it being 2.7K, it leads me to think that's what it should be! If you accept space being 0,or just above 0K naturally, then the stars etc would heat it up slightly to aprox 2.7K!! Very simplistic, so please tell me why that cant be the case.  Scientists talk of the bb coming from a quantum fluctuation in pre-bb space/void. We know there are quantum fluctuations now, but does that mean there could be before the bb? Aren't we just putting what we know of our universe now, onto a pre bb era? Currently we cannot say anything about a pre bb era? Isn't it just using a mechanism, that we know of, that seems to 'fit the bill'???  In science we use cause and effect, how/why things work. Any knowledge found can always be followed by a further question. Do you think that, by definition, a universe that started from a bb leads to a dead-end in science? Obviously, we are far away from that point, but in theory, do you feel that it would make what we could learn finite? I'm not a big believer in the bb as the ultimate 'start' of the universe, although i believe it/ or the physics of it, play some part in our universe. To this, i have a follow-up question if you'd permit me, following your answers to the previous questions. Thankyou Courtney very much for your time.  Regards Richard.

Answer
(Finally have time to finish the answer. See the additional material below the dashed line.)

As far as dark matter is concerned, it seems reasonable to me to suppose that it could represent an extra amount of normal mass, as the distribution of dark matter matches that of visible matter. That is, where you see lots of visible mass, there is lots of dark matter, and where you don't see much visible mass, there appears to be negligible amounts of dark matter. (You might want to read my discussion of Dark Matter in Galaxies, at http://cseligman.com/text/galaxies/dark.htm for a discussion of why we are certain that dark matter exists in galaxies, and why we have no idea what it is.)

Unfortunately, that doesn't mean that we're just wrong about the masses of the visible objects. The orbital motions of binary stars firmly establish the masses of such systems, and of course the orbital motions of the planets establish the mass of the Sun. A comparison of the masses determined in this way for different types of stars leads to the Mass-Luminosity Diagram (see http://cseligman.com/text/stars/mldiagram.htm), which pretty firmly ties down how much mass can be ascribed to the easily visible stars. There could be (as noted in the page on Dark Matter in Galaxies) a lot of mass in stars which are simply too faint and too far away to notice, but whether that is correct cannot be determined with current technology, so although it is a reasonable possibility, it isn't one which is being seriously pursued.

There is also the question of just how much dark matter there is. According to theories of the origin of the Universe, there is a limit to how much "normal" matter can exist without changing the composition of the Universe from what we observe, to something completely different. It is barely possible that the dark matter in galaxies might be normal matter in some form which is simply difficult to observe (as noted above). But comparisons of the characteristics of the CMB to theories of the origin of the Universe, particularly when combined with studies of the expansion rate in different eras (from studies of type Ia supernovae observed at different distances), suggest that the mass of the Universe is two to three times the mass of the galaxies, including any dark matter inside them. There is some uncertainty in the numbers (probably more than anyone is willing to admit), but it looks like there must be a substantial amount of dark matter which is not in any way like normal matter. What it is like depends upon who you talk to. My opinion is that none of the current theories of what dark matter is (or is like) are satisfactory, but that doesn't keep them from being popular.

However, given that caveat, it does appear that the visible portion of the Universe (all stars and galaxies more or less easily observable, plus an estimated fraction of additional material based on extrapolations of samples of things that are harder to observe) is only a few percent of the so-called "critical mass", and no more than a fifth to a third of the mass of the galaxies themselves. Of the extra, non-visible mass, half or more of what is in the galaxies could be normal matter in a difficult to observe form, but part could be some kind of "strange" dark matter, which is what people generally think of, when they use the term dark matter. And if the mass of the Universe is two or three times that of the galaxies, even counting the dark matter in them, then practically all of that extra mass must be "strange" dark matter.

In other words, if the mass of the Universe is about the same as that of the galaxies, counting their dark matter, your supposition that it is simply more "normal" mass might be correct, although barely so; but if the Universe has a mass anywhere near what it is thought to have, then the bulk of the dark matter is something which we (1) don't know what it is, (2) don't know what it is like or how it behaves, and (3) have absolutely no idea how to determine either of those.

As a final note for dark matter, I should note that there cannot be any significant amount of dark matter in the solar system, unless it is buried inside the Sun. As it happens, there are popular theories which propose just that; but if they are correct, the dark matter located in the Sun (or any other star) would not count toward the dark matter that makes the Universe more massive than the stars, because it would already be counted, in determining the masses of the stars. So when we talk about dark matter in the galaxy, or the Universe as a whole, as being something other than what is in the stars, we mean "in addition to any which is inside individual stars".

Basically, the dark matter has to be small massive objects which are too faint to observe, lying between the visible stars (this was the reason for proposing brown dwarfs, as discussed in Dark Matter in Galaxies), OR it has to be some kind of infinitely thin "gas" of unknown nature which is scattered between the stars (and the galaxies), which does not significantly interact with any kind of electromagnetic radiation, but does interact with massive objects, through the force of gravity. The amount of such stuff inside our solar sytem (other than inside the Sun, if there is some inside the Sun) must be less than a trillionth of a percent of the mass of the Sun, because if there were that much or more lying within the Solar System, it would noticeably affect the orbits of the planets. But the Solar System is very small compared to the distance between stars, so even if the dark matter was so thinly spread that in a region the size of our Solar System there was less than a trillionth of a percent of a Solar mass, there could be tens of Solar masses of dark matter lying between us and the nearby stars -- which is exactly what we would need to fit the observations.

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The CMB was predicted by the original Cosmic Fireball theory (later mockingly derided as "just a Big Bang theory of everything"), and calculations that the redshift caused by the expansion of the Universe should cause the (actually several hundreds of millions of degrees temperature at the end of the Fireball) radiation to be red-shifted to appear like black-body radiation of just under 3 Kelvins was part and parcel of the original theory. After the discovery of the actual microwave background (I believe in the late 50's, but it would be easy for you to look up the exact date) by scientists at Bell Labs, a number of subsequent studies have nailed down the temperature as 2.7+ Kelvins, exactly identical to black-body radiation of that sort, and in recent years, having fluctuations in brightness and temperature whose magnitude and angular size match the predictions of theories which suppose that at the moment of the Big Bang, the entire Universe (currently observable and whatever lies beyond that) was a point mass with its current mass and energy content, but a size less than the so-called Planck size.

As to what caused this remarkable event, that's another matter. But in the 1930's Paul Dirac created a theory of quantum mechanics which proposed that if quantum mechanics gave some statistical probability that something or other could happen, then at some point in space and time, such a fluctuation COULD and in rare circumstances WOULD occur, albeit with a very low frequency, corresponding to the probability involved. You might read a discussion of pair production, which is the original "proof" of the accuracy of his theory, for a detailed explanation of how this works, but to put it accurately but crudely, at any given point in space-time, there is a certain probability (generally very, very low) that the zero energy expected to exist there (we are talking about an "empty" point in space-time) might not actually be zero, but some non-zero value. Keep in mind that in defining a point in space-time, we require that its size in both space and time be so small that it is impossible to measure what is going on there without using probes whose own energy would produce an uncertainty in the resulting measurement larger than what is "actually" going on there. Given that, if you saw something happening in such a point of space-time, you couldn't say that there was some violation of the conversation of (the presumably zero) mass/energy, because it might be an inaccurate measurement, caused by the "observer's" interference.

What Dirac proposed is that if there was some very small probability of a "glitch" in the energy of a point in space-time, sufficient to do something interesting (such as creating an electron-positron pair), then such things would occur (very rarely), at a rate corresponding to how low the probability was. And since that is indeed what we observe in "low-energy" cases such as pair production, it could conceivably occur in higher-energy cases, as well.

The result of this is discussed on my web page, "In the Beginning", at http://cseligman.com/text/prologue.htm which discusses how, in some prior Universe, any given point of empty space-time should have a nearly zero probability of having a glitch with an energy equal to the entire mass and energy of a brand new Universe. The chance of that happening at any given place and time is negligible. But there is a huge amount of space and time in a Universe, and taking every point in both space and time into account, such a huge number of space-time "points" that no matter how small the chance of such an unimaginable glitch occuring at one of them, it is inevitable that it will occur at some of them.

Now, it is important to note that there is no way to verify this directly. In the instant (the single point of space-time involved) that a Universe-size energy glitch occurs at some point in the original Universe, a whole new space-time is created, which is completely separate from the original Universe. So there is no hint that anything has happened, from the viewpoint of any denizen of the original Universe. BUT in the "new" Universe, what you get is identical to the conditions believed to exist in the Big Bang which presumably started our Universe (hence the note on my webpage about this being called "the ultimate free lunch", or in more elegant terms, "Ex nihilo, omni -- Out of nothing, everything".

In other words, quantum mechanics predicts that although such events are unlikely, they are inevitable. And cosmology predicts that if they occur, they would produce a CMB which looks exactly like what we observe. So it seems likely that the Big Bang was not a unique event, save for our own particular Universe, and that there is an infinity of Universes, each of which is creating an infinity of new Universes, but none of which is in any way observable from any of the others. (There is a sort of soap bubble analogy used in diagrams of this, which gives the impression that the soap bubbles might be able to intersect and interact; but that is a poor representation of the presumed reality, in which there is an infinite amount of space between each Universe and all the others, because each one creates a completely new spacetime, completely separate from all the others, and any representation of one Universe's spacetime would have no hint of any other spacetime, either prior or subsequent.)

In the discussioins above (from earlier today, and right now) I have referred to several pages on my website, which may help to better illuminate the concepts discussed here. The other discussions are in some cases longer, and in other cases shorter; but in general, they are better organized than the above, since I have rushed through it to try to make up for the nearly day and a half between receiving your note, and finally having the time to "properly" answer it. Hopefully, between what is here and what is on my website, you can get a clear answer to your questions; but if you have ANY doubts or additional questions, please feel free to ask away, and hopefully next time, I'll be able to give you a more immediate reply. I just had a confluence of work, prior obligations, and medical emergencies which conspired to keep me from properly addressing your answer in a timely manner, and although there was nothing I could do about that, I do apologize for not getting back to you sooner.