Constraints and Signatures in Particle Cosmology

By Mark Trodden | June 26, 2007 7:09 am

If you are really lucky, then you may have a great new idea about particle physics. It may be a way to address the hierarchy problem (why is gravity so much weaker then the known particle physics forces), or to generate mass for fermions (after all, we haven’t found the Higgs yet), or to understand the flavor hierarchy (how come there are three repeated families of particles in the standard model with increasing masses), or perhaps to unify all the forces into one (Grand Unification). Obviously, your obligation is to begin systematically computing the consequences of this idea for existing and future particle physics experiments.

Thirty years or so ago, with a few notable exceptions this would have been the end of the story. But it has become increasingly clear to most physicists that there exists a complementary list of consequences that should be figured out; those for cosmology. These days, this approach is basically second nature to any of us who might have new ideas about how the micro-world works, and reflects the modern thinking that particle physics and cosmology are not distinct disciplines, but are two sides of the same set of questions.

So, parallel to the cross-section and decay rate calculations, what are the most common cosmological areas in which one currently looks for further constraints on one’s new particle physics idea? What new questions do you need to ask yourself?

  1. Does your theory contain any new long-lived elementary particles? If it does then you better watch out and you better beware. You see, such particles may quit interacting with other species in the relatively early universe (if they are weakly-coupled) and so maintain a rather high abundance as the universe cools. Because of this, a relatively straightforward calculation shows that they can rather quickly become the dominant contributor to the matter content of the universe. This can be a real disaster, given how much we know about the cosmic expansion history, and is to be avoided. The couplings, masses and lifetimes of such particles therefore need to be such that they either never dominate the energy budget of the universe, or make just the right contribution to be interesting (see my second list below).
  2. A related problem can arise if your theory contains long-lived particles that are too light, because if there are too many of them around when structure is trying to form, then because they are light they typically move at relativistic speeds and stream through overdense regions smoothing them out and ruining structure formation.
  3. Does your theory contain any new topological defects, such as monopoles, domain walls or cosmic strings?. If the vacuum structure of your particle physics theory is sufficiently topologically complex, then any symmetry breakings that occur may lead to trapped regions of false vacuum that cannot decay. If so, then many of the constraints mentioned for long-lived elementary particles may apply to these objects. In addition, some topological defects can form networks that redshift more slowly then matter, coming to dominate at a later time in the universe, or can generate a spectrum of gravitational radiation that is in conflict with our detailed measurements of the timing of the millisecond pulsar. If this last constraint is a problem, then it is also possible that the defects unacceptably distort the spectrum of the Cosmic Microwave Background radiation (CMB).
  4. In the early universe, does your theory significantly alter either the matter content or the expansion rate of the universe during the formation of the light elements – Big Bang Nucleosynthesis (BBN)? This can be an immediate death blow, since the remarkable agreement between measurements of the abundances of the light elements and those predicted within the standard cosmology is one of our triumphs and our earliest direct test of the Big Bang model.
  5. Going further back in time, does any of the new physics in your model lead to new sources for density (or metric) perturbations? If so, when you process these through cosmic history, what does the resulting spectrum of the CMB look like, and how does it correlate with the related prediction for the spectrum of large scale structure? What about the expected results of weak lensing studies? How do all of these compare with the wonderful data that has poured in over recent years?

If your big new idea passes all these tests (and others I haven’t mentioned) then you may really have something. If this is all there is to it, then you can be happy that your new construction gives rise to novel particle physics phenomena, while remaining safe from cosmological constraints.

However, one might be able to do better. While our underlying model of cosmology is in remarkable agreement with our ever-increasing stream of data, there are a number of critical areas where we are, no pun intended, in the dark. It may be that your new idea can help with some of these genuine cosmological conundrums. What should you look for? While the list is increasingly long these days, here are some common ideas.

  1. Got WIMPS? There are lots of connections between new particle physics (particularly beyond the standard model physics addressing the hierarchy problem) and dark matter. Perhaps you have a dark matter candidate in the theory. You’ll need to check to see if there is a long-lived (stable for all intents and purposes) particle with couplings of the appropriate strength (weak, or below) and mass in the right range. And it needn’t be a WIMP (Weakly Interacting Massive Particle). Maybe there’s an axion, or even a WIMPZilla.
  2. There are a number of hints that the highest energy cosmic rays may require exotic new physics for a complete understanding. Above a certain energy (the Greisen-Zatsepin-Kuzmin (GZK) cutoff), particles from cosmological distances shouldn’t reach us at all, because they would scatter off the CMB. This has led people to speculate that any ultra high-energy cosmic rays (UHECRs) may be a signature of new particle physics. Does your theory contain any particles or phenomena that could allow this to happen, and what spectrum of UHECRs should we expect? Some of those topological defects I mentioned above may be an example.
  3. You don’t, by any chance, have any unnaturally weakly coupled heavy scalars out there do you? Because we’re looking for an inflaton to do all the early universe’s heavy lifting. Your candidate should be able to quasi-exponentially expand the universe, flatten out its spatial hypersurfaces, causally connect seemingly unconnected regions of the microwave sky, generate all the matter content of later epochs (reheating) and imprint upon it the density perturbations necessary to seed our observed large scale structure.
  4. Come to think of it, there isn’t an alternative mechanism to inflation in your theory is there? It is fair to say that inflation is our best current idea about what happened in the early universe, but is not without its problems, and an attractive competitor would be very welcome. Good luck though – that list of requirements is pretty hard to satisfy.
  5. Now, generating the matter is one thing, but you’ll typically create an equal amount of antimatter, which will annihilate with matter, and leave very little left over to form all that lovely structure, never mind us. What you really need is a way to create an asymmetry between matter and antimatter (in fact baryons and antibaryons are what we care about) – a baryogenesis mechanism. Perhaps your inflaton candidate is exotic enough to generate this as part of reheating. Perhaps there are asymmetric decays of heavy particles in your theory, or maybe a way to make nonperturbative baryon number violating transitions work. You should get on that right away!
  6. The 800 pound gorilla in the room these days is, of course, cosmic acceleration. Do you address the cosmological constant problem? If not, is there a dark energy candidate in your model? This one would be wonderful, but don’t stress if you don’t have anything to add here – you’re in good company.
  7. Actually, since we’re now on to things that seem incredibly difficult to explain, your theory isn’t able to tell us why there are 3+1 (space+time) dimensions is it? That would be just great.

As you can see, modern cosmology has yielded a lot of hurdles for any up and coming particle theory to cross. It’s a tough new world out there. On the other hand, look at all the macroscopic problems your new microscopic theory may be able to address. The above lists certainly aren’t exhaustive – I have definitely missed out important constraints – but, more importantly, there are probably other crucial particle-physics connections out there with which to constrain theoretical ideas, just waiting to be discovered, perhaps by you!


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About Mark Trodden

Mark Trodden holds the Fay R. and Eugene L. Langberg Endowed Chair in Physics and is co-director of the Center for Particle Cosmology at the University of Pennsylvania. He is a theoretical physicist working on particle physics and gravity— in particular on the roles they play in the evolution and structure of the universe. When asked for a short phrase to describe his research area, he says he is a particle cosmologist.


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