Sunday, 6 November 2005

Redshift: the Quantum Explanation

Last year I attended a course about cosmology given by Prof. Raul Abramo at the Physics Institute of the University of São Paulo. At some point, he was explaining the Hubble´s law and how we could calculate the Hubble constant by measuring the (cosmological) redshift of far objects. The explanation for the redshift is that, as the universe expands, it stretches the electromagnetic waves so that their wavelength is increased. A larger wavelength means a lower frequency and then the light is redshifted, or equivalently, the frequency of a light wave emitted from an object goes in the direction of the red light and beyond (to the infrared and more…) as its frequency is lower than the frequency of blue light.

After the class, I was thinking about the effect and realized that I was told a classical (not quantum) explanation. The nature of light seems to be quantum and then a full quantum explanation should exist. I first asked for a former professor of mine, Prof. Henrique Fleming, and he told me that there was no official explanation, because we haven´t a Quantum Gravity theory yet, and the redshift was a relativistic, therefore a gravitational effect.

I thought about the question and arrived at the conclusion that the explanation probably would be given by an interaction between the photon and the graviton. Somehow, the photon should interact with the graviton and give it some part of its energy. As energy is proportional to the frequency, less energy means less frequency and a redshift. Then, last week, I saw in Physics Forums a comment about a preprint by Michael Ivanov entitled “Low-energy quantum gravity” that presented the idea in detail. I did not read the paper yet, but it seems that it is an interesting one. Although nobody is sure that the graviton really exists (and if you go to Physics Forums you will see a lot of people that will say that it didn´t), maybe as a low-energy approximation the concept could explain the redshift effect in a quantum mechanical way, something that was not done till now.

Picture: Barred Spiral Galaxy NGC 1300 - NASA

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