This week I’m studying a very nice article on the ArXiv by L.Perivolaropoulos on recent observational challenges to the ΛCDM model (thanks Bee).
The ΛCDM model is the technical name given to the concordance model of Big Bang cosmology (see final post in cosmology 101 series). Essentially, the model is the best attempt to account for the three main strands of observational evidence: the measurements of the cosmic microwave background, the measurements of the large scale structure of the universe by gravitational lensing, and the supernova measurements of the accelerated expansion of the universe. CDM stands for Cold Dark Matter, the postulate that much of the matter holding the galaxies and galaxy clusters together is unseen – i.e. does not couple with the electromagnetic interaction (see previous post on Dark Matter). Λ refers to the so-called cosmological constant – i.e. the ‘dark energy’ term thought to be responsible for the current acceleration of the universe expansion (see previous post on dark energy here).
The matter-energy composition of the universe according to ΛCDM
However, cosmologists are well aware that there is an alternative: the ΛCDM model could simply be wrong, and the postulates of dark matter and dark energy completely spurious, if our underlying theory of gravity – general relativity – does not apply at the largest scales. Both postulates arise from the attempt to shoehorn the observational data into gravitational theory, and it is always possible that the underlying theory is incomplete (after all, we know GR breaks down at the smallest scales). There is a very nice discussion of this in Perivolaropoulos’ s paper, in the context of six experimental observations that have emerged in the last few years that don’t seem to fit easily into the ΛCDM model.
Of course, given the spectacular success of general relativity in explaining so many aspects of our universe so far, the betting money is on relativity being correct, while the new observational data may modified as more measurements are made (this has happened countless times before). Either way, it’s a really nice update on the current state of play and shows how good science is done – not to mention the usefulness of the ArXiv database.
Update
Over on the DiscoverScience blog, Sean Carroll also has very nice post on a specific challenge to the concordance model from measurements of the large scale structure of the universe by weak gravitational lensing. Again, both the post and the discussion afterwards are excellent and give a good idea of how this sort of science is done.
It”s worth mentioning that both dark matter and dark energy are favourite targets of skeptics, philosophers of science and other commentators. To be sure, they both probably seem like an obvious fix to an outsider, particularly given their postulated prevalence relative to ordinary matter (our universe is estimated to comprise 73% dark energy, 23% dark matter and only 4% ordinary matter!). However, in this sort of debate, it’s important to listen to the experts. While keeping an open mind, most cosmologists seem convinced that dark matter almost certainly exists. The general line is that you can see it – by its gravitational effect, not electromagnetic. This is perfectly feasible if dark matter is made up of WIMPS (weakly interacting massive particles), a not unreasonable proposition. Such particles may even be detected at the LHC, which would be very exciting. It should also be remembered that the existence of dark matter is also invoked to account for the nucleosynthesis of the elements, a seperate plank of the big bang model. Finally, there are now strong experimental hints of the existence of dark matter from studies of galaxy collisions
Evidence for dark matter in the bullet cluster
As for dark energy, it is certainly true that this is a lot more speculative, and could turn out to be one of many different things (see wiki for a good summary). However, it’s important to note that the postulate does not arise solely from the supernova measurements – there are also indirect evuidence of dark energy from measurements of the cosmic microwave background.
Antimatter 
The problem, I think, is not the theory (general relativity) but the cosmological principle asumed that is blatantly opposite to experimental data: the universe is not homogeneous. One cannot take as data conclusions based on a hypothesis (the standard cosmological model) denied by the experimental data. It’s only a good approximation now clearly insuficient to understand the observational cosmos.
good point!hard to evaluate at what point this approximation becomes invalid…
One question:
Does not the current standard model consider the universe to be flat and infinite and homogeneous (in the big scale)?
But, if this is the spacetime solution to Einstein equations considered valid, then spacetime has to be infinite from the very beginning (and with flat space). On the other side the universe is considered also to come from a state of infinite curvature/density and extremely hot. But if we have a spacetime with that characteristics how’s it possible for it to be flat and infinite in volume? Should we assume an infinite space full from the beginning and flat even with almost infinite densities? Doesn’t black holes prove that space around an infinite density singularity is not flat at all?
These things I can’t put together in my mind. Maybe you can help me.
Thanks!
Dr Cormac O’Raifeartaigh said
“… both dark matter and dark energy are favourite targets of skeptics, philosophers of science… in this sort of debate, it’s important to listen to the experts.”
Experts have a vested interest in the standard model and I find them far from being fair-minded when it comes to critiquing a alternate theory that that goes against the core of their beliefs. Six difficulties with the LCDM model and Rachel Bean’s findings are alluded to. What is not mentioned is the failure to detect dark matter underground despite years of effort and millions spent.
My alternate gravity theory is based on the belief that heat rather than mass mediates the gravitational force. The Stephan-Boltzmann law tells us that if mass has temperature it will have heat leaving it the form of radiation. The heat leaving mass is as ubiquitous as the mass itself. Scientist have never done experiments as I have to see if the heat transferred through a test is gravitationally attractive. To see five experiments which show that the weight of a test mass will increase or decrease by 2-9% depending on the direction with which the heat transfers through the test mass go here http://vixra.org/abs/0907.0018.
Since there is high correlation between the mass of a star and its luminosity, it is difficult to tell whether the star’s mass or its luminosity is doing the attracting. The Ptolemaic geocentric theory had the same problem. It is difficult to tell whether or not the heavens revolve around the earth in a 24 hour period or this appearance is just due to the earth revolving on its axis every 24 hours.
“While keeping an open mind, most cosmologists seem convinced that dark matter almost certainly exists.” I claim that the Pioneer anomaly, the flyby anomaly, and Milgrom’s Law suggest the validity of the Rañada-Milgrom effect, which is that the -1/2 in the standard form of Einstein’s field equations should be replaced by -1/2 + sqrt(15) * 10**-5. If M-theory with neutralino physics explains dark matter, then the Rañada-Milgrom effect implies that the neutralinos would be WIMPs that are weirdly gravitationally interactive — contradicting the standard theory of cold dark matter. If the Gravity Probe B science team took into account the Rañada-Milgrom effect, then would the problems with their 4 gyroscopes be an indication of quantum gravitational waves and not “misalignment torques”?