Today’s e-mail conversation!

Unlike what Dr. Manhattan believes, it turned out that the Universe does notice sometimes!
The Jornal da UNESP wants to publish an article on the recent discovery of BICEP2 in their next issue for the public audience. They’ve interviewed me last week and also asked me to write a couple of paras on my research. I just copy-paste a part of the email conversation between the journalist (in red) and me.
I think the main question I have is about the importance of gravitational waves. The ones that were created in the early universe can be used to study the origins and the process of development of the universe (please correct me if I’m wrong). What about the ones that you study, which are created by binary systems of black holes and super massive compact stars? Do they also help on the study of the “history of the universe”? Can they provide other kinds of information as well? And how?

Excellent question! Before I answer your question I would like to make it clear that, in fact, there are not different kinds of gravitational waves BUT different sources! They are all gravitational waves with the same intrinsic properties. For instance, they all have the same speed and same polarization modes while they might have different extrinsic properties, like different amplitude and frequency range and that’s because they might have different origins. For example, the gravitational waves coming from the early universe are in a much lower frequency band compare to ones emitted by compact binary systems.

The importance of direct detection of the gravitational waves by LIGO and other interferometric, ground-based observatories mainly includes the two following items. First, direct detection will provide new tests for the fundamental theory of gravity. A part of my research is focused on studying the general properties of gravitational waves in specific theories of gravity (like Einstein’s general theory of relativity or its alternatives like Scalar-Tensor theories of gravity) no matter where they come from, early universe or compact binary systems! The general properties of gravitational waves only depend on the ultimate theory of gravity that rules the gravitational interactions all around the Universe. Although Einstein’s general theory of gravity has passed all the performed tests successfully but its validity as an ultimate theory of gravity is still seriously questionable from several aspects including quantum gravity. Beside Einstein’s general theory of gravity, many viable alternative theories of gravity has been proposed that they have also passed all the performed tests too. We can not distinguish the predictions of one theory from another by any of the earlier tests in the weak gravitational field regime where the mass sources are relatively far apart and the gravitational fields are relatively weak (e.g. solar system tests). One of the most important motivations for direct detection of gravitational waves is extending the tests of fundamental theories of gravity to the strong field regimes where the mass sources are close to each other and since then the gravitational fields are strong (like the wild coalescence of two compact stars). Gravitational wave signals from such strong field regimes are potentially useful to distinguish the predictions of Einstein’s theory and its alternatives.

Second, direct detection of gravitational waves by LIGO (and other gravitational wave observatories) in the near future will open a totally new window to our understanding of the Universe. Gravitational-wave astronomy provides new tools to explore the universe out there that we have never been able to reach before by any of our earlier astronomical telescopes such as optical, radio, x-ray, and gamma-ray telescopes. For example, with LIGO we can measure intrinsic astronomical parameters of the compact sky objects like the mass and spin of neutron-stars and black-holes at very far distances along with extrinsic parameters like sky-localization and distance, with relatively good approximations. As a member of LIGO collaboration, a part of my research is focused on data analysis of gravitational waves. The gravity group of ICTP-SAIFR, composed of Riccardo Sturani and myself, officially joint LIGO collaboration in October 2013.

Also, I’d like to ask you if you have any images of your work which you think would be good to explain/visualize gravitational waves, or that would just be good images to ilustrate the article.

I think the best picture that you can use might be the following. Credit: NASA.


Caption: Gravitational waves can be found in a wide range of frequencies from very low frequencies (coming from the early Universe) to higher frequencies (emitted by compact binary systems). Different types of detectors (on the bottom of the above picture) are needed to detect gravitational waves in each range of frequencies (colorful spectrum in the middle) that are emitted from different sources (on the top). The recent detection of gravitational waves, claimed by BICEP2 collaboration, covers only a small range of the frequency spectrum (as you can see in the picture at bottom-left). LIGO and other terrestrial interferometers aim to detect gravitational waves in the other end of the spectrum.

PS_If you think above picture is too technical for public audience you may use the following attached picture and say something like: gravitational waves are the ripples in the fabric of space-time.


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