Search for the Missing Universe

Chris Gondek writes "The Sydney Morning Herald has reported that one of the greatest discoveries of our time could be made under the Yorkshire moors. Deep in a Yorkshire mine, scientists are toiling to solve a cosmic puzzle that has baffled astronomers for 70 years: about 90 per cent of the universe is missing. Analyse the movements of stars and you can work out how much matter is making them swirl round in galactic islands and how much makes galaxies cluster together as they do - in other words, you can work out how much mass makes the universe look the way it does. But measurements suggest that the universe is not what it appears."

Comments

Historical Analogues

[Farley Mullet]

Back in the 19th century, astronomers had noticed that there was a minute procession in the perihelion of Mercury (in other words, the point in Mercury's orbit that is closest to the sun kept moving forward) that they couldn't account for using the Keplerian/Newtonian model of celestial dynamics. Astronomers thought that it must have reflected the influence of some massive, distant unknown planet; predictions were made about where this planet was and what its mass was, but astronomers couldn't find it. Then all of a sudden General Relativity came along, and our understanding of mechanics in gravitational fields was improved, and the procession was easily predicted (within an incredibly small margin, as I recall). So it seems just as likely that the "missing mass" is due to a theoretical deficit as it is due to an observational deficit.

Re:Historical Analogues

[krlynch]

While this is a useful cautionary tale, you have to be a bit careful in your interpretation of it. It is quite easy to show (and it is a typical undergraduate classical mechanics homework problem) that the perturbations of Mercury's orbit CAN NOT be explained within the Newtonian model by the addition of another point source (ie, a planet), because any such explanation would cause a larger than observed perturbation to the orbits of Venus, Earth, and Mars. And this was a well known issue BEFORE Einstein started working on his GR theory. In other words, physicists knew there was something wrong with the theory long before they had a theoretical solution, because the preminent gravitational model of the time was predicting the wrong thing when confronted with the available data.

The cautionary aspect of the tale, though, is well understood by the larger physics community, and dozens of modified and new models of gravity HAVE been proposed in the literature to explain the apparent "missing mass" of the universe without invoking unobserved particles; but they all run afoul of some observation or other. The current model has been arrived at by the consensus of a large number of physicists and astronomers around the globe over a long span of time ... it isn't a flash in the pan, and while it could be wrong, the data on many length and time scales just seems to get more compelling as we add to it, rather than less.

In this case, we understand GR, its cosmological implications, and the requisite post-Newtonian approximation schemes well enough that we have developed a model that match ALL known observations with the inclusion of dark matter and dark energy components. It isn't just one or two observations of rotation curves that have pushed us in the direction of dark matter, but literally dozens of observations, from widely different length and time scales, from cosmic background radiation to rotation curves, from earthbound laboratory measurements to interstellar radiotelescope observations. It is certainly POSSIBLE that there is a theoretical description available that doesn't require dark matter/energy, AND explains all of the data, but it looks more likely to the daily practitioner that the current theory is good at the length scales it is being applied to, and the dark matter/energy is the simpler solution.

I'd like to point out one other cautionary tale to those who want to blame the theory, and points out that well tested theories are not tossed out immediately when new or contradictory data comes along: in the early part of the 20th century, observations of beta decay led many physicists to conclude that the very fundamental conservation laws of energy and momentum (and the entire theoretical framework that so neatly explains them) would have to be tossed out the window, because the observed decay products (electrons and nuclei) didn't appear to follow those conservation laws. But some very smart people, including Pauli, said "Wait, the theory has worked so well up to now that we should look for a SIMPLER explanation; we propose a to-date unobserved particle with no charge and no mass produced in association with electrons in these decays." That was scoffed at by many, but a few years later just that very particle was observed: the electron neutrino. My point is just that, while you need to keep an open mind and be willing to challenge both experiment AND theory, you have to do so with the WHOLE picture in mind, and not just a tiny corner .... that is what science and the scientific process is all about.

Stephen Hawking's wishful thinking

[kindofblue]

In his book, A Brief History of Time, I think he said something to the effect that he believes that we'll figure out most of the big questions about the nature of the universe within 10 years or so. That was about 15 years ago. Does anybody remember reading this?

When I saw that, I remember thinking that's naive and contrary to the entire history of scientific research. Anyway, it reminds me that even some of the best minds say some of the stupidest things. Especially in physics.

I'm not a physicist but I'm pretty damn sure that Stephen Wolfram and Roger Penrose have had some pretty wacky theories when they venture away from straight physics, like into cellular biology, free will, philosophy, ...

Not True

[efuseekay]

Not true.

Gravity is only tested to solar system scales, and in an indirect way, galactic cluster lensing effects.

At very large scales, say of the Hubble radius, we have no tests of gravity. Cosmological models are almost always based on the belief that Gravity works at the very large scales, an extrapolation of many orders of magnitude. There is no proof that this is a valid extrapolation, and there are hints that they are not. (Like they lead to an extremely highly unlike situation. Check out This Talk )

Large scale modifications of gravity may affect the smaller scales, but these effects are naturally suppressed (you can cook up theories where they are not suppressed, but then it is not "large scale" modifications anymore). So to discover these effects are hard.

We have experimental constraints of course, but they are not very strong.

Re:Thought...

[gilroy]

Blockquoth the poster:

Anybody ever stop to consider that the other 90% must have some sort of structure?

There is no logical necessity for that. Although you can conceive of matter that exists subject to a host of strong interactions but which does not interact with us -- that there are two "classes" of matter that exist separately -- there is no evidence for that. Occam's Razor says, don't invent whole universes for the heck of it. The simplest explanation consistent with the facts is taken to be true.

Indeed, many of the "hot" dark matter theories presume exactly no structure to the dark matter ... just streaming neutrinos flashing throughout the volume of the universe.