Research Interests: Pulsars!

What are pulsars? and... Why?

When the cores of massive stars run out of nuclear fuel they collapse catastrophically - a supernova. This compresses the core material so hard that even its atoms are destroyed, forming an incredibly dense ball of neutrons: a neutron star (henceforth NS).

Figure 1: The Crab Nebula is the remnant of a supernova that occurred in the constellation Taurus in the year 1054 AD (as seen from Earth). Credit: HST/NASA/ESA.

Pulsars are NSs that seem to pulsate in radio and gamma rays. As we discuss below, they are superb tools for studying the basic laws of the Universe. The discovery of radio pulsars in 1967, by Jocelyn Bell, demonstrated that neutron stars exist in the Universe, and that they are the end products of the lives of massive stars. This fundamental discovery resulted in a Nobel prize in Physics for A. Hewish, in 1974.

Recycled radio pulsars possess extraordinary long-term rotational stability. Timing their radio pulses, we can determine their spin periods, spin-down rate, position and proper motion with spectacular accuracy.

Because of their formation, most recycled pulsars are found in binary systems. In these cases, can use radio pulsar timing to measure their orbital motions with unique, and astounding precision.

Most of the companions to recycled pulsars are also compact objects: other NSs or white dwarfs. This means that the components behave as point masses. This has a fundamental consequence: the small relativistic effects predicted by GR are not mixed with any Newtonian effects caused by tides or the rotation of the companion. This means that those relativistic effects become in principle detectable, particularly for the more compact binaries. In some cases the precision of the timing is such that we can actually measure those effects to very high precision. We thus have not only a very precise experiment, but an extremely clean one as well.

The Hulse-Taylor binary pulsar

The best known example of what can be achieved by timing binary pulsars is the study of the first binary pulsar, PSR B1913+16. The system was discovered in 1974 by Russel Hulse and Joseph Taylor using the Arecibo radio telescope. With a spin period of 59 ms, the pulsar is recycled, its companion is another NS, and the orbit is very tight (its period is 7 hours and 45 minutes) and eccentric (e = 0.61).

In this system, three relativistic effects on the orbit were measured from the pulsar timing. The first two (the advance of periastron and the Einstein delay) were used to determine the masses of the two NSs assuming GR. The third relativistic effect, a decrease of the pulsar's orbital period, matches exactly the GR prediction for the orbital decay for this system caused by the emission of gravitational waves!

This measurement confirmed the self-consistency of GR for strongly gravitating objects - something not possible in the Solar System. But more importantly, it confirmed experimentally the physical reality of gravitational waves (GWs) - they indeed carry energy across space as predicted by GR. This indirect detection of GWs happened decades before LIGO.

Because of the importance of their discovery, Russel Hulse and Joe Taylor were awarded the Nobel Prize in Physics in 1993.

The promise of gravitational wave astronomy

This is a supremely important discovery since gravitational waves are not merely another relativistic effect which we can use to test GR. They are messengers: they carry information across vast distances. That means that if we build an extremely sensitive GW detector, we can in principle detect faraway objects by listening to the gravitational waves they emit.

Would such a detector really hear anything at all? This is where the full importance of the discovery of PSR B1913+16 becomes clear. The orbital decay of the system implies that it will continuously lose orbital energy, with the two neutron stars inevitably merging in 330 million years. At that time they will emit a gigantic GW burst. If we observe several double NS systems in our Galaxy, there must be many, many other such systems in our Galaxy and the Universe, with many of them merging in any particular year. This means that a sufficiently sensitive detector should certainly hear many such mergers. This process is so prevalent in the Universe that it is actually though to originate most of the heavy elements in the priodic table, like gold, platinum, lead and uranium!

It is because of this certainty that LIGO and Virgo could be built. LIGO's detection of the merger of two 30-solar mass black holes in 2015 opened up a new window on the Universe. The 2017 detection of GW170817 showed that the ground-based detectors are already sensitive enough to detect the mergers of double NS systems like PSR B1913+16.

Following the direct detection of gravitational waves with LIGO, Rainer Weiss, Barry Barish and Kip Thorne were awarded the Nobel Prize in Physics in 2017.

Research Projects

The main body of the research I have been involved in (see my list of refereed publications) builds upon the technical and conceptual legacy from the work on PSR B1913+16 to greatly expand the scope of the science. The main questions are:
  1. How precisely does GR describe gravity?

  2. Are there other good descriptions of gravity?

  3. What is the true nature of gravitational waves?

  4. How massive can NSs be?
Apart from this, I have also contributed with improved techniques for the analysis of pulsar data.

I have also been deeply involved in a variety of pulsar surveys which were important to find many of the systems mentioned here: the ALFA and AO327 pulsar surveys at Arecibo and the HTRU-North pulsar survey with the Effelsberg telescope.

1. How precisely does GR describe gravity?

General relativity - the current, unsurpassed description of gravity - appears to be incompatible with quantum mechanics. Furthermore, it predicts its own failure at the centres of black holes; this particular finding shows that it cannot be the ultimate answer concerning gravity.

More recently, alternative theories of gravity have been proposed to explain the rotational curves of galaxies and clusters of Galaxies without the need for Dark Matter, the accelerated expansion of the Universe without the need for Dark Energy and Cosmic Inflation (fundamental for understanding the origin of the Universe), which was driven by a still unknown force.

Is gravity really responsible for these phenomena, or are we in the presence of other forces of nature? Clearly, understanding it is important not only for understanding the laws of Nature, but also the origin, evolution and contents the Universe!

Pulsar experiments like the one mentioned above for PSR B1913+16 have tested GR with very high precision: The last two systems are the most relativistic double NSs known in our Galaxy, even more extreme than the double pulsar (see Fig. 2). They will certainly lead to very precise tests of GR in the near future!

Figure 2: Comparison between the sizes of the Sun, the ``classical'' pulsar - NS systems (The Hulse-Taylor system, B1913+16, and the ``Double Pulsar'' system, J0737−3039) and their tighter, more relativistic cousins, respectively J1757−1854 and J1946+2052, published in 2017 and 2018.
Credit: Norbert Wex.

All current tests from double neutron stars show that GR provides a very good explanation of all observed phenomena.

2. Are there other good descriptions of gravity?

Clearly, gravity seems to behave in a way that is similar to GR. However, the viable alternatives to GR also pass the extremely precise tests posed by double NS systems.

If gravity behaves in a way that is different from GR, how can we find out?

The answer is that while all viable metric theories of gravity predict the Einstein equivalence principle (EEP) - as they must, given the very strong experimental constraints on this - they differ from GR regarding the strong equivalence principle (SEP): unlike GR, all viable alternatives predict its violation! Therefore, the most likely phenomena to observe beyound GR should be consequences of SEP violation.

SEP violation would have several observable consequences, mostly a) the polarization of orbits of a binary system in the presence of the gravitational field of a third object (the Nodtvedt effect) and b) the emission of dipolar gravitational waves (DGW) in compact, asymmetric binary pulsars. We have looked for these effects in several systems: One of the neat advantages of these tests is that even if we don't falsify GR, we falsify other theories that would predict observable amounts of SEP violation and associated effects in these systems!

See here a review on tests of GR with astrophysical observations, which includes a review of the pulsar tests.

3. What is the true nature of gravitational waves?

As mentioned above, the observations of pulsar - NS systems like PSR B1913+16 confirmed the existence of gravitational waves decades before LIGO's detection. With current results from PSR J0737−3039 we are already able to test GR's predictions for the emission of quadrupolar GWs much more precisely: The theory is correct to within 0.16%!

The tests with asymmetric systems like PSR J1738+0333, J2222−0137, J0348+0432 and PSR J1913+1102 strongly constrain any dipolar component to gravitational wave emission for a diversity of NS masses. Thus, they represent not just tests of particular gravity theories, like general relativity and Scalar-Tensor gravity; they are physics experiments that, in a theory-independent way, probe deeply into the fundamental nature of gravitational waves.

These pulsar results are strongly complementary to results on NS-NS and NS-BH mergers from ground-based observatories.

4. How massive can NSs be?

At the centre of a NS, matter is denser than atomic nuclei. For that reason, its composition and behavior is not known. This is a fundamental problem in nuclear physics.

Figure 3: For each EOS (named in the figure) the relation between mass and radius for all NSs is indicated by its related curve. Pulsar mass limits are indicated by the horizontal lines. Figure created by Norbert Wex. EOSs tabulated in Lattimer & Prakash (2001) and provided by the authors.

One of my main research topics is the measurement of NS masses. The maximum NS mass constrains the macroscopic behavior of super-dense matter, in particular the relation between density and pressure (known as the equation of state, or EOS). This is how it's done: Each EOS predicts a family of NSs that is represented by a curve in figure 3 above. If a particular EOS predicts a maximum mass smaller than the largest measured NS mass (horizontal lines are 95% lower limits for the three most massive pulsars, i.e., there is a 5% probability that each is below its respective line) then it is excluded. Look here for a review on NS masses, radii, and how they constrain the EOS.

Improved techniques for the analysis of pulsar data

Apart from the scientific studies above, I have contributed with new techniques for the analysis of pulsar data: Imprint / Privacy policy / Back to Paulo's main page.