2023-06-30 06:00:10
Several PTA (Pulsar Timing Array) scientific collaborations around the world have joined forces to announce the discovery of a new class of very low frequency gravitational waves. These very slow oscillations of space-time (The concept of space-time was introduced in the early 1900s and resumed…) would be caused by pairs of super-massive black holes, but could also come from various energy processes in the early Universe.
Stanislav Babak, research director at APC, member of the EPTA collaboration and current chairman of the IPTA steering committee (International Pulsar Timing Array), sheds light on this announcement.
Image: Danielle Futselaar
Why is this discovery an important scientific advance?
Like other PTA collaborations, we observe strong evidence of the existence of the gravitational wave signal in the frequency band (A frequency band defines a range of frequencies that have…) nano-Hz. We are not yet claiming that it is a detection. We still need to perform additional analyzes on the combined data of all PTA collaborations, to obtain an indisputable result.
If this discovery is confirmed, it will open a new frequency window (Ultra-low frequency between 1 and 100 nano-Hz) in the astronomy (Astronomy is the science of observing the stars, seeking to explain…) gravitational waves. When Karl Jansky ushered in the era of radio astronomy (Radio astronomy is a branch of astronomy dealing with the observation of the sky in the…) in the 1930s, many discoveries came to enrich our understanding of universe in which we live. That is what is about to happen with this new discovery.
What types of cosmic events can be studied with this new window?
There are several sources of gravitational waves in this ultra-low frequency band. The most promising are supermassive black hole binaries, ranging in mass from millions to billions of solar masses. The set (In set theory, a set is intuitively a collection…) of the population of these pairs of supermassive black holes orbiting each other must produce a stochastic signal of gravitational waves which should be noticeable. We might also see the signal from individual binaries if they are relatively close.
There are other possible sources of gravitational wave signals in this band, in particular gravitational waves from various energetic processes in the early Universe. Nevertheless, the exact interpretation of the source of these gravitational waves could prove very difficult today.
APC scientists have considered surprising origins for these gravitational waves. Could you tell us a bit more?
Mikel Falxa (former doctoral student (A doctoral student is a committed beginning researcher, under the supervision of a director of…) at the APC, currently a postdoc at the CNRS in Orléans) studied the case of close binaries and established in one of the ETPA articles that this signal could come from a binary black hole in the Fornax cluster. It could also be a stochastic gravitational wave signal. Hippolyte Whateverjay, a doctoral student at the APC, directs the analysis in this direction. In this case, we have three alternatives (Alternatives (original title: Destiny Three Times) is a novel by Fritz Leiber published…) to the scenario of supermassive black hole binaries: the signal could be generated by a network of strings cosmic; it could also come from the early Universe when protons and neutrons emerged from the primordial plasma; or it could be primordial gravitational waves generated during the initial inflation of the Universe. All of these scenarios could potentially produce a noticeable signal. This is the subject of the fourth and fifth articles prepared by the EPTA collaboration.
Tell us a little more about the detection technique used
For several decades, radio telescopes around the world have been monitoring ultra-stable millisecond pulsars. They are very old neutron stars that emit a narrow beam of light (Light is the set of electromagnetic waves visible to the eye…) radio from their magnetic pole (A magnetic pole is a point of ” convergence” of the magnetic field lines present…). When the beam passes through (A sleeper is a fundamental part of railway tracks. It is a piece laid across…) the radio telescope (A radio telescope is a specific telescope used in radio astronomy for…), it is perceived as a “pulse”, similar to the signal from the headlight. Hence the name “pulsars”. We are interested in pulsars whose rotation is extremely stable and whose pulses arrive at very regular intervals. More precisely, we observe the very slight variations which modify the interval between the pulses, due to certain effects which impact the time of arrival of the pulses. Gravitational waves also induce such deviations, but in a very specific way.
Thanks to long-term observations of radio pulsars and sophisticated data analysis techniques (Data analysis is a subfield of statistics concerned with …), we try to distinguish the effect of gravitational waves. In particular, gravitational waves induce a correlated signal in the data from each pulsar, but this signal is weak and difficult to distinguish from noise. In short, we measure gravitational waves by their interaction (An interaction is an exchange of information, affects or energy between two agents within…) with electromagnetic signals as they propagate from pulsars towards the Earth, as we do with the Virgo and LIGO interferometric detectors, but there we use lasers and mirrors.
What is the next step for the collaboration?
The next step will be to combine our data (EPTA+InPTA) with similar data from NanoGrav (North American collaboration), PPTA (Australian collaboration) and complement them with MeerCat and MeerTime data (precursors of the Square Killometer Array). This will be the most sensitive data to date and it is already partially ready. They will be analyzed jointly by all the PTAs under the aegis of the IPTA (International PTA). It is very likely that the analysis of these data confirms the detection of gravitational waves. If not, we will need longer observations. 65% of EPTA data comes from the Nancay radio telescope. We have very good receivers and continue to monitor the pulsars.
In the near future: FAST – Chinese radio telescope (already in operation), and SKA (Square Killometer Array) will provide very high quality data, which will improve the sensitivity of EPTA by at least an order of magnitude.
Will this technique replace terrestrial interferometers?
No, LIGO, Virgo and Kagra observe between 10 and 2000 Hz (high frequencies), the future LISA space mission (launch in 2035) will operate from 0.1 to 100 milli Hz (low frequencies). PTA is complementary, it observes between 1 nano Hz and 100 nano Hz (ultra-low frequencies). These projects are complementary to each other.
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