A new study says that gravitational waves emitted when distant black holes collide and merge, causing the very fabric of space-time to ring like a bell, can be used to help measure the expansion rate of the universe.
Since the late 1990s, astronomers have realized that the universe is not only expanding, but that it is doing so at an accelerating rate. The cause of the so-called delayed-time acceleration has remained a mystery, and has been given the placeholder name “dark energy. ”
The researchers were baffled by the fact that the two main methods for measuring cosmic expansion give different values for the rate, which is called Hubble constant. this The shadow of contradictioneven as both methods have become more accurate over the years.
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These two techniques are the “late time” method, which takes into account the speed of galaxies and their distance from us, and the “early time” method, which studies “fossil light” after the great explosion Call Cosmic microwave background.
Late time measurements currently give an expansion rate of about 73 ± 1 km/s per megaparsec, while early measurements give a value of 67.5 ± 0.5 km/s per megaparsec.
This prompted scientists to search for a supporting method for measuring the Hubble constant. This is where the new study comes in.
Gravitational lenses and gravitational waves
The new study suggests using a predicted phenomenon Albert Einstein It is usually associated with the distortion of light called gravitational lensing to measure the Hubble constant.
gravitational lens It is an effect that originates from Einstein The general theory of relativity. The great physicist’s 1915 theory of gravity predicts that mass has a warping effect on space and time, uniting as a four-dimensional entity called spacetime.
This warp means that when light from a background source passes an object of great mass such as a galaxy, its path is bent by a “gravitational lens”. This effect can magnify a background source, and is used to great effect for seeing early galaxies by observatories such as NASA James Webb Space Telescope (JWST).
Gravitational lensing is usually associated with light. But gravitational waves—ripples in space-time caused by the acceleration of objects of enormous mass, such as two black holes spiraling toward each other—should be affected similarly. this means gravitational waves From these violent merging events should also exhibit gravitational lensing, as does light.
Light can take different paths following a lensed object because the amount it deflects depends on how close it is to the gravitational lens. This means that the light reaches the Earth at different times, and this time delay can cause the same object to appear in multiple places in a single image. Since gravitational waves can also take different paths after gravitational lensing, they should also display a similar arrival time delay, meaning that gravitational-wave detectors could, in theory, detect gravitational waves from the same event at different times.
Study team members said this can be used as a measure of the Hubble constant. That’s because the rate of expansion of the universe affects the distance between the sources of gravitational waves – Black hole Mergers, for example — and the galaxy that warps space-time and acts as a gravitational lens and distance to Earth.
The team said that the amount of gravitational wave reflection should depend on the expansion rate of the universe, and therefore on the Hubble constant. They suggest that the larger Hubble constant would lead to a larger portion of lenticular black hole mergers and also smaller values of the time delay than would be seen in the case of the smaller Hubble constant.
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The advantage of measuring the expansion rate of the universe with gravitational waves rather than light is that these ripples are not affected when they pass through huge clouds of gas and dust, whereas light can be absorbed or change its frequency. This means that this technique could allow astronomers to “see” more into the history of the universe than even ultralensed light would allow.
Scientists have not yet detected a strong effect of gravitational lensing on gravitational waves generated by merging black holes, and the technique proposed by the team will be based on a catalog of thousands of gravitational wave events, which are not yet available. They were the first gravitational waves It was first discovered in 2015, so this is still a new field of science. However, major developments are afoot.
The sensitivity of terrestrial gravitational wave detectors has improved with the recent significant improvements of Gravitational-Wave Laser Interferometer Observatory (LIGO), Virgo, and the Kamioka Gravitational Wave Detector (KAGRA). In addition, the first space gravitational-wave detector, the Europa Laser Interferometer Antenna (LISA), is scheduled to be launched in 2037.
With these improved tools, scientists can begin to build a database that allows observations of gravitational lensing in gravitational waves. In this data, the team expects to find a small fraction of repeated signals from the same black hole merger events, just as the same distant light sources appear multiple times in JWST images due to gravitational lensing.
“A major science goal for future detectors is to provide a comprehensive catalog of gravitational wave events, and this will be a completely new use of this impressive data set,” Santa Barbara, USA. he said in a statement.
The research was published June 30 in the journal Physical review letters.
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