Space telescopes may be needed to finally solve the cosmology crisis

Gravitational-wave (GW) observatories were a great addition to cosmologists’ arsenal in the decade of the shortage. With their first effective detection at the Laser Interferometric Gravitational Observatory completed in 2015, they have opened up a new world of data collection for scientists. However, so far, they have not solved one of the fundamental problems at the heart of their discipline: the “Hubble tension”. Now a new paper discusses the possibility of using a network of new space-based gravitational-wave observatories to get closer than ever to the true value of one of the most important numbers in the Universe.

Edwin Hubble didn’t actually discover “Hubble’s Law,” the equation containing the constant that bears his name — that work was done before and independently by Alexander Friedmann and George Lemaitre. Their work showed that the Universe was expanding and that the rate at which it was growing seemed to be determined by the distance between the observer and the galaxy itself.

Now commonly accepted as the expansion of the Universe, this was a revolutionary theory in the 1920s when it was first formulated. However, like many good scientific theories, it can be simplified to a single equation: v = H0D. In this case, v is the speed of separation (the expansion of the Universe), D is the distance to the galaxy being compared, and H0 is known as the “Hubble constant”.

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Anton explains another potential difficulty in finding the true Hubble constant: it may not be constant at all.
Credit – Anton Petrov YouTube channel

The Hubble constant has been a source of discussion for years, as its value will literally help determine the fate of the Universe. If it’s large, the Universe will end up in heat death, where galaxies are so far apart that they can never interact. Alternatively, if it is small, the Universe could end up in a “Big Bounce” where gravity overcomes the expansive force of the Universe. Eventually, everything is brought back to one lonely point, just like another Big Bang.

Its importance gives scientists plenty of reasons to worry about the Hubble constant, but an exact number is notoriously difficult to pin down, and various experiments have resulted in some variation in the reading. It has never reached an accuracy threshold that the scientific community is willing to accept, commonly believed to be within 0.9%. In particular, two popular measurement methods, the cosmic microwave background measurements and the distance ladder method, do not agree on the value.

Gravitational waves may resolve this “Hubble tension” at the heart of cosmology, according to a new paper by researchers at Northeastern University in China. There has recently been a wave of new science following the detection of the first gravitational wave. However, this fundamental new type of tool could make many more discoveries. But even the results from the most sensitive detectors here on Earth wouldn’t be able to fully constrain the Hubble constant to a point where its value could finally be ascertained.

UT video on the expansion of the Universe.

So why not put sensing platforms in space? This has several advantages, including less interference from ground sources of jitter (i.e. earthquakes), but more importantly, they can coordinate together. Several gravitational-wave observatories are already scheduled for launch in the coming decades. Taiji, TianQin, and LISA are all observatories that, when wired together, can detect gravitational waves at the millihertz level. These are typically formed by the merger of massive black hole binaries, and sometimes these events are joined by emission in the electromagnetic (ie, light) spectrum. Combining the detection of gravitational waves by gravitational observatories with a haphazard EM signal could help find and constrain the Hubble constant in a way not possible before.

That dream is still a long way off, as none of the space-based gravitational-wave observatories are functional yet, and some won’t be for at least a decade. However, when they come online, cosmologists will surely be interested in what data they can collect regarding this most important number and perhaps humanity will finally know the fate of the Universe.

Learn more:
Jin et al – The Taiji-TianQin-LISA network: precise measurement of the Hubble constant using bright and dark sirens
UT – Is the Hubble constant not constant?
UT – Astronomers have a new way to measure the expansion of the universe
UT – Hubble’s law

Main picture:
Artist’s impression of the LISA mission.
Credit – NASA

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