Gravitational waves strike the Earth again: GW151226

The LIGO and Virgo collaborations announced today, June 15, at the 228th meeting of the American Astronomical Society in San Diego, a new gravitational wave event. Simultaneously, an article is published in Physical Review Letters.

The event was observed on December 26, 2015 at 3.38.53 UTC in the two LIGO detectors of Livingstone and Hanford (1.1 millisecond later). The event, interpreted as the merger of two black holes, is not as bright as the one announced last February, and thus the signal is not as spectacular:


One of the reasons is that the black holes are not as massive as in the “discovery” event GW150914: the two masses are 14 and 8 solar masses, and the final black hole mass is 21 solar masses. The analysis is using templates of mergers predicted by theory, and comparing the signal with them. The signal to noise ratio (the quantitative way physicists express the fact that the signal stands out of the background “noise” in the detector) is computed to be 13, to be compared with 24 in the case of GW150914.

The distance of the event is estimated to be 1.4 billion light years.

Because the signal lasts longer in the detector than the first event observed, the LIGO-Virgo collaboration could determine that one of the initial back holes (and the final one) was spinning.

In the press conference at the AAS meeting, the collaboration reminded everyone of another event (already mentioned in the original discovery paper) which is presumably another black hole event merger. They call it LVT151012, where LVT stands for LIGO Virgo Trigger (it was observed on October 12 2015). The signal to noise ratio is 9.7 and the collaboration does not feel confident enough to call it a discovery. If it corresponded to a black hole merger, the mass of the final black hole would be 35 solar masses.

If the announcement of February 11 was closing one hundred years of quest for gravitational waves, the announcement of today clearly opens the era of gravitational waves astronomy.

We will have a great occasion to talk about this new discovery in the hangout held this Thursday 16 June, as a conclusion of the Gravity! course second session.

Why should I care about what goes on beyond the horizon?

Why should I care about what goes on beyond the horizon?
Just think of the horizon on the sea. It is a circle around you which is fictitious, but important to you as an observer: you cannot see beyond it. You might then say: OK, my universe is a disk i.e. the surface of the sea within the horizon, and I do not care about what is beyond it because I cannot see it. This is all good until, one day, you spot a ship appearing at the horizon. This tells you that you should care about what is beyond your horizon, and also that your Universe is not a disk, but is curved.

Well this is much the same about the cosmological horizon, except that there are 3 dimensions, not the 2 dimensions of the surface of the sea, so the horizon is a sphere in space, not a circle. Moreover, we are in space-time, so the horizon is a time-dependent notion, it changes with time. Thus, by waiting for some time (not on a human scale but on a cosmological scale), you might see some astrophysical objects entering your horizon, like ships do at sea. So you should care about what is beyond your present horizon.