Windows to the Earliest: Neutrinos and Gravitational Waves

We continue our series of excerpts (and discussion) from the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.

Thesis B7…
Neutrinos and gravitational waves will in principle allow us to peer back to much earlier times (the time of neutrino decoupling and the quantum gravity era respectively), but are much harder to observe at all, let alone in useful directional detail.  Nevertheless the latter has the potential to open up to us access to eras quite unobservable in any other way.  Maybe they will give us unexpected information on processes in the very early universe which would count as new features of physical cosmology.

The cosmic microwave background (CMB, T = 2.73 K) points us to a time 380,000 years after the Big Bang when the average temperature of the universe was around 3000 K.  But there must also exist abundant low-energy neutrinos (cosmic neutrino background, CNB, CνB, relic neutrinos) that provide a window to our universe just one second after the Big Bang during the radiation dominated era.  That’s when neutrinos decoupled from normal baryonic matter.

As the universe expanded, these relic neutrinos cooled from a temperature of 1010 K down to about 1.95 K in our present era, but such low-energy neutrinos almost never interact with normal matter.  Even though the density of these relic neutrinos should be at least 340 neutrinos per cm3 (including 56 electron neutrinos per cm3 which will presumably be easier to detect), detecting them at all will be exceedingly difficult.

Neutrinos interact with matter only through the weak nuclear force (which has a very short range), and low-energy neutrinos are much more difficult to detect than higher-energy neutrinos—if they can be detected at all.  If neutrinos have mass, then they will also interact gravitationally with other particles having mass, but this interaction is no doubt unmeasurable due to the neutrino’s tiny mass and the weakness of the gravitational force between subatomic particles.

The cosmic gravitational background (CGB) points us to the time of the Big Bang itself.  Faessler, et al. (2016) state

The inflationary expansion of the Universe by about a factor 1026 between roughly 10-35 to 10-33 seconds after the BB couples according to General Relativity to gravitational waves, which decouple after this time and their fluctuations are the seed for Galaxy Clusters and even Galaxies. These decoupled gravitational waves run since then with only very minor distortions through the Universe and contain a memory to the BB.

References
Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.
[http://arxiv.org/abs/astro-ph/0602280]

Faessler, A., Hodák, R., Kovalenko, S., and Šimkovic, F. 2016
[https://arxiv.org/abs/1602.03347]

The Sachs-Wolfe Effect

The cosmic microwave background (CMB) peaks at a wavelength of 1.9 mm and frequency 160.23 GHz, if spectral radiance is defined in terms  of frequency.  If spectral radiance is defined in terms of wavelength, then the CMB peaks at wavelength 1.1 mm.  This radiation comes from all directions, and the curve of intensity as a function of wavelength very closely approximates a perfect black body having temperature 2.725 Kelvin.  Since the Big Bang 13.8 billion years ago, the universe has expanded and cooled so that today its temperature is 2.725 K.

About 380,000 years after the Big Bang, the universe had expanded and cooled enough so that for the first time it became transparent to electromagnetic radiation.  Thus when we accurately map the exact spectrum of the cosmic microwave background in different directions, we can construct a “baby picture” of the universe when it was only 380,000 years old.

Our baby picture is not smooth but has features.  At that early time, the universe had already developed into denser regions, and less dense ones. Now, it is important to note that cosmic microwave background photons that left a denser part of the universe have been gravitationally redshifted to slightly longer wavelengths (and lower frequencies) to a greater extent than elsewhere.  This is called the Sachs-Wolfe effect.

 

Lots of exciting cosmological information is coming out of mapping the tiny differences in the CMB spectrum as we look in different directions. I’m wondering, though, if anyone has seen temporal variations in the CMB? In other words, if you stay pointed in a particular direction and carefully measure the CMB spectrum over time, does it change or fluctuate at all (after all sources of noise have been removed)?  Even though our current understanding of cosmology might lead us to believe that the CMB would not change fast enough for us to measure, has anybody looked?

Small Universe

We continue our series of excerpts (and discussion) from the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.

4.3.1 Small universes
A Small Universe: a universe which closes up on itself spatially for topological reasons, and does so on such a small scale that we have seen right round the universe since the time of decoupling.  Then we can see all the matter that exists, with multiple images of many objects occurring.  This possibility is observationally testable by examining source statistics, by observation of low power in the large angle CBR anisotropies, and by detecting identical temperature variation on various circles in the CBR sky.  There are weak hints in the observed CBR anisotropies (the lack of power on large angular scales) that this could actually be the case, but this is not solidly confirmed.  Checking if the universe is a small universe or not is an important task; the nature of our observational relationship to the universe is fundamentally different if it is true.

In 1900, Karl Schwarzschild (1873-1916) was perhaps the first to suggest the idea of a small universe topology that would lead to multiple images of the same object at different points in the past.  Though most cosmologists favor the idea of a very large universe with a simple topology, the possibility of a more complex small universe topology is still not out of the question.  The universe might be measurably finite in some or all directions.

The smaller a finite topological region of space, the easier it should be to discover multiple images of the same object at different ages (except for CMB features which will all be the same age).  The distribution of distant sources might show “patterns” that are related to more nearby sources.  A comprehensive survey of sources at redshifts between about z=2 to z=6 is still needed before any conclusions can be drawn.

Another approach, of course, is to look at patterns in the CMB temperature (intensity) and polarization.  Analyses of the most recent release of Planck satellite data, however, shows no evidence of a compact topology smaller than our visual horizon.

References
Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.
[http://arxiv.org/abs/astro-ph/0602280]

Luminet, J.-P. 2016,  arXiv:1601.03884v2 [astro-ph.CO]