Using Cosmology to Establish the Quantization of Gravity

A short article that I’ve read today (via @seanmcarroll).

arXiv:1309.5343 [hep-th]

Title: “Using Cosmology to Establish the Quantization of Gravity”

Authors: Lawrence M. KraussFrank Wilczek

Here is the abstract:

While many aspects of general relativity have been tested, and general principles of quantum dynamics demand its quantization, there is no direct evidence for that. It has been argued that development of detectors sensitive to individual gravitons is unlikely, and perhaps impossible. We argue here, however, that measurement of polarization of the Cosmic Microwave Background due to a long wavelength stochastic background of gravitational waves from Inflation in the Early Universe would firmly establish the quantization of gravity.



Continuing with chapter #20 of the course “Dark Matter, Dark Energy: The Dark Side of the Universe” by Prof. Sean Carroll, now it is time to talk about “Inflation“. I remember when I was studying General Relativity at University that we didn’t spend much time on this concept. I thought by that time that it was a bit odd and also and old-fashioned idea but it has turned out to be a crucial theory (as it is seems to be quite stablished among many cosmologists) for the understanding of the very first moments of evolution of space-time, of our Universe.

As always, some of the concepts/ideas/people… whatever that Prof. Carroll has mentioned in this chapter are collected here for further reference (at least for myself):

  • Inflation: already mentioned in this previous post. It refers to a extremely short phase of the evolution of the Universe, at the beginning of the Big Bang, in which the Universe could have expanded exponentially fast, rapidly transforming curved space into flat one.
  • Alan Guth (1947 – ): american physicist that proposed the inflationary hypothesis in 1980.
Spectacular realization

Guth’s logbook showing the original idea of Inflation. December 7, 1979.

  • Inflaton: scalar field postulated to be the responsible of the rapid expansion of the Universe, known as inflation.
  • Reheating: this is a poorly understood process by which the temperature of the Universe prior to the inflationary phase gets back to its previous values. It is also known as thermalization. The reheating consists on a decay of the inflaton field into particles and radiation, starting the radiation dominant phase.
  • Multiverse: the multiverse is the hypothetical set of multiple possible universes or bubble universes that are popping into and out of existence and colliding all the time, with the space between them rapidly expanding.
  • Monopole problem:  Grand Unified Theories propose that at high temperatures, such as the ones taking place in the early universe, stable magnetic monopoles would be produced. Nevertheless, this heavy particles, which ought to be present today, haven’t been observed in nature so this is an open question in these theories. Here comes inflation to solve it: if a period of inflation occurred below the temperature where magnetic monopoles could separate from each other as the universe expands, the density of these particles would be highly lowered by many orders of magnitude and this could be the reason why there’s no track of them at the moment.
  • Horizon Problem: this referes to the problem of determining why the Universe appears to be homogeneous and isotropic. In a Big Bang model without inflation we couldn’t explain why two widely separated regions of the observable universe have the same temperature.
  • Flatness Problem: this referes to the problem of determining why the density of matter in the universe is comparable to the critical density necessary for a flat universe (Euclidean),  as recent observations of the cosmic microwave background have demonstrated. Inflationary theory solves this problem as it forces the universe to be very flat (to a very high degree, I mean).
  • Polarization of the CMB (Cosmic Microwave Background): one of the predictions of the inflationary universe is that primordial gravitational waves were created during the inflation era. These waves can be accessed by measuring the CMB polarization. Experiments to detect these perturbations are ongoing.

Although I recommend purchasing the original videos from The Teaching Company, this chapter can be seen on YouTube here (part 1) and here (part 2) and here (part 3).

Measuring and Understanding the Universe

This is the last article that I have read from, also (like the previous one) a 2003 article:


Title: “Measuring and Understanding the Universe”

Authors: Wendy L. FreedmanMichael S. Turner


Revolutionary advances in both theory and technology have launched cosmology into its most exciting period of discovery yet. Unanticipated components of the universe have been identified, promising ideas for understanding the basic features of the universe are being tested, and deep connections between physics on the smallest scales and on the largest scales are being revealed.


I have decided to underline the most relevant ideas or terms that I consider are worth keeping in mind. As I have mentioned somewhere else in this blog (in case I haven’t, this is the moment), I want to note down the important concepts, key words, ideas, etc., that interest me. Again, this blog site is not intended to be a place where every single concept will be explained. For that, there are many other webs… and, as I always say, in any case Wikipedia (like Google Translator, if needed) is our friend.

(I’ll try not to repeat this kinda disclaimer in future posts, I promise 😉

Some the ideas / key items that I have underlined are:

  • Inflation: enormous expansion of the Universe that took place in a tiny fraction of a second during the early history of the Universe. A concept introduced by Alan Guth in 1980.
  • Ordinary matter (baryonic): the kind of matter that accounts for a 4% of the total-mass density in the Universe. Neutrons and protons.
  • Dark Matter: about the total 30% of the total-mass energy of the Universe, composed of particles most likely formed in the early Universe. Something fundamentally different from the ordinary matter we are made of.
  • Dark Energy: 2/3 of the total mass-energy of the Universe whose repulsive gravitational effects began causing the expansion of the Universe to speed up. If it didn’t exist, it is thought that the flat Universe we live in (yeah, it is supposed to be flat!) would decelerate its expansion by its own self-gravity and recollapse (Have you heard of the Big Crunch? It was one of the possibilities for the evolution of the Universe, when I was studying GR at university… Now we know more of the story… The Universe is expanding, but not only expanding… It is expanding faster and faster and faster each time…).
  • Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological model: an exact solution of Einstein’s field equations of general relativity that explains quite well our Universe. It assumes that in large scales the Universe is homogeneous and isotropic.
  • Currently known components of the Universe: ∑ (ordinary matter of baryons + cold dark matter + massive neutrinos + cosmic microwave background and other forms of radiation + dark energy).
  • Expansion of the Universe: discovered in 1929 by Edwin Hubble.
  • Hubble constant: slope of the relation between distance and recession velocity, a relation that Hubble established when he measured the distance to some nearby galaxies. There have been many measures of this constant, as the one obtained from the results of the WMAP satellite that estimated a value of : Ho ~= 71 km/sec/Mpc (Mpc stands for megaparsec, a unit of length). Here is a picture to give some color to this post: a well-known image obtained by the WMAP (I bet you have seen this one before).
7 year WMAP image of background cosmic radiation (2010)

7 year WMAP image of background cosmic radiation (2010). Image from the Wikimedia Commons.

  • Big-Bang Nucleosynthesis (BBN): early phase of the Universe in which light elements were formed by means of protons and neutrons combination.
  • Measurements of the CMB (Cosmic Microwave Background): the CMB is the resulting radiation left over from an early stage in the development of the Universe. It provides the best explanation for the accounting of ordinary matter in the Universe.
  • Cold Dark Matter (CDM): descriptive theory that tries to explain how the structure of the Universe plausibly arose. CDM is a hypothetical form of matter that interacts very weekly with electromagnetic radiation. Leading candidates for the CDM particle are axions and neutralinos.
  • Axion: a CDM hypothetical particle a trillion times smaller in mass than that of the electron.
  • Neutralino: a hypothetical particle predicted by supersymmetry that might fit well as the CDM hypothetical particle. It is supposed to be a hundred times more massive than the proton.
  • Baryogenesis: the three conditions proposed by Sakharov necessary for the Universe to develop a slight excess of baryons over antibaryons: i) action of microscopic forces that do not conserve the net number of quarks; ii) breaking of the symmetry between particles and antiparticles; iii) departure from the thermal equilibrium.


WELL, that’s all for know, folks. ‘See’ you in future posts!