Will Gravitational Wave Detectors in Space Discover New Force Fields in the Universe?

Gravitational Waves

 

New research has shown that future gravitational wave detections from space will be capable of finding new fundamental fields and potentially shed new light on unexplained aspects of the Universe. There are four known fundamental interactions or forces in the Universe: gravitation, electromagnetism, the weak interaction, and the strong interaction.LISA’s Unprecedented Accuracy

Professor Thomas Sotiriou director of the University of Nottingham’s Center of Gravity and Andrea Maselli, researcher in theoretical physics at the Gran Sasso Science Institute (GSSI) and Istituto Nazionale di Fisica Nucleare (INFN) associate, together with researchers from SISSA, and La Sapienza of Rome, showed the unprecedented accuracy with which gravitational wave observations by the space interferometer LISA (Laser Interferometer Space Antenna), will be able to detect new fundamental fields. 

In this new study researchers suggest that LISA, the space-based gravitational-wave (GW) detector which is expected to be launched by the European Space Agency (ESA) in 2037 will open up new possibilities for the exploration of the Universe.

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New Fundamental Fields –Extending Known Laws of Physics

Sotiriou explains: “New fundamental fields, and in particular scalars, have been suggested in a variety of scenarios: as explanations for dark matter, as the cause for the accelerated expansion of the Universe, or as low-energy manifestations of a consistent and complete description of gravity and elementary particles. We have now shown that LISA will offer unprecedented capabilities in detecting scalar fields and this offers exciting opportunities for testing these scenarios.” 

“Scalar fields have been introduced in several extensions of General Relativity or of the Standard Model, and tend to couple with the metric tensor. Such fields are often thought as possible explanations of dark energy/dark matter problems, or may be conceived as a classical limit of quantum gravity theories,” Andrea Maselli told The Daily Galaxy.

Somewhat simplistically, the hypothesized scalar field can be understood as an extension of the known laws of physics, beyond the currently known fields or interactions governing the behavior of matter and energy, such as the gravitational and electromagnetic fields. 

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Deviations from General Relativity

Observations of astrophysical objects with weak gravitational fields and small spacetime curvature have provided no evidence of such fields so far. However, there is reason to expect that deviations from General Relativity, or interactions between gravity and new fields, will be more prominent with strong gravitational fields, causing much larger spacetime curvatures.

For this reason, the detection of GWs — which opened a novel window on the strong-field regime of gravity — represents a unique opportunity to detect these fields.

“The small-curvature regime has been widely tested and doesn’t seem to show evidence of such fields. The high curvature, highly non-linear regime looks the most favorable to probe the existence of new fields coupled to gravity,” Maselli told The Daily Galaxy.

Extreme Mass-Ratio Inspirals

Extreme Mass Ratio Inspirals (EMRI) in which a stellar-mass compact object, either a black hole or a neutron star, inspirals into a black hole up to millions of times the mass of the Sun, are among the target sources of LISA, and provide a golden arena to probe the strong-field regime of gravity, where gravity strongly affects light passing next to a massive compact object, like a massive black hole with millions of times the mass of the Sun, and even causes the light to be captured by the object. 

The smaller body performs tens of thousands of orbital cycles before it plunges into the supermassive black hole and this leads to long signals that can allow us to detect even the smallest deviations from the predictions of Einstein’s theory and the Standard Model of Particle Physics.

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The researchers have developed a new approach for modeling the signal and performed for the first time a rigorous estimate of LISA’s capability to detect the existence of scalar fields coupled with the gravitational interaction, and to measure how much scalar field is carried by the small body of the EMRI. Remarkably, this approach is theory-agnostic, since it does not depend on the origin of the charge itself, or on the nature of the small body. The analysis also shows that such measurement can be mapped to strong bounds on the theoretical parameters that mark deviations from General Relativity or the Standard Model.

The Last Word

In an email to The Daily Galaxy, physicist and co-author Paolo Pani,  Associate Professor of Theoretical Physics at Sapienza University of Rome, member of the Gravity theory and gravitational wave phenomenology Group, wrote:

“General Relativity (GR) has been brilliantly confirmed across several scales, including those now accessible by gravitational-wave astronomy. However, several theoretical arguments strongly suggest that GR is an incomplete theory that should be extended at high energies (that is, large curvatures). The main motivation is that GR is inconsistent from the point of view of quantum field theories (technically: it is not renormalizable) and modifying it in the regime of high curvatures might cure these problems. Modifying GR at high curvatures almost inevitably leads to introducing new fundamental fields that “mediate” the new interaction. There are several examples in string theory, for instance the dilaton, moduli fields, dark photons, etc. These fields are negligible in regimes of small curvatures (e.g. Solar System or supermassive black holes), but can be important in other regimes.

“An important figure to keep in mind is that the curvature at the horizon of a BH of ten solar masses is about 10 billion times larger than that near a BH of a million solar masses. Therefore, light BHs are the best to probe high-curvature corrections to GR. This is why extreme mass-ratio inspirals (EMRIs) might be very convenient to probe these theories in a model independent way: in EMRIs a stellar-mass BH or a neutron star orbits around a supermassive BH, making million of orbits in the bandwidth of the future LISA mission. In this class of theories the supermassive BH (low curvature) is unaffected by the new fields, so the orbits of the small secondary object are the same as in GR. However, the small secondary object is affected by the new field and acquires a “charge”, similar to the electric charge of the electron. As such, while orbits it emits extra radiation so its inspiral towards the central object proceeds (typically) faster than in GR. LISA will be extremely sensitive to this effect because it will observe these systems for over one year.

“Probably the nicest feature of this model stands in its generality: virtually all higher-curvature corrections to GR will show this effect and LISA will be able to search for it in a model-independent way.”

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LISA will be devoted to detect gravitational waves by astrophysical sources, It will operate in a constellation of three satellites orbiting around the Sun millions of kilometers far away from each other. LISA will observe gravitational waves emitted at low frequency, within a band not available to terrestrial interferometers due to environmental noise. The visible spectrum for LISA will allow to study new families of astrophysical sources, different from those observed by Virgo and LIGO, as the EMRIs, opening a new window on the evolution of compact objects in a large variety of environments of our Universe.

Source: “Detecting fundamental fields with LISA observations of gravitational waves from extreme mass-ratio inspirals” by Andrea Maselli, Nicola Franchini, Leonardo Gualtieri, Thomas P. Sotiriou, Susanna Barsanti and Paolo Pani, 10 February 2022, Nature Astronomy. DOI: 10.1038/s41550-021-01589-5

Avi Shporer, Research Scientist, with the MIT Kavli Institute for Astrophysics and Space Research via Paolo Pani,  and University of Nottingham

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