According to the leading scenario, our Universe contains only a few percent of ordinary matter. One quarter of the cosmos is made of the elusive dark matter, which we can feel gravitationally but not observe, and the rest consists of the even more mysterious dark energy — first discovered about 20 years ago — that is driving the current acceleration of the Universe’s expansion.
This model is based on a multitude of data collected over the last couple of decades, from the Cosmic Microwave Background (CMB) — the first light in the history of the cosmos, released only 380,000 years after the Big Bang and observed in unprecedented detail by ESA’s Planck mission — to more ‘local’ observations.
The latter include supernova explosions, galaxy clusters and the gravitational distortion imprinted by dark matter on distant galaxies, and can be used to trace cosmic expansion in recent epochs of cosmic history — across the past 9 billion years.
The new study, published in the journal Nature Astronomy, points to another type of cosmic tracer — quasars — that would fill part of the gap between these observations, measuring the expansion of the Universe up to 12 billion years ago.
The new technique uses ultraviolet (UV) and X-ray data to estimate the quasar distances.
In quasars, a disk of matter around the supermassive black hole in the center of a galaxy produces UV light. Some of the UV photons collide with electrons in a cloud of hot gas above and below the disk, and these collisions can boost the energy of the UV light up to X-ray energies.
This interaction causes a correlation between the amounts of observed UV and X-ray radiation. This correlation depends on the luminosity of the quasar, which is the amount of radiation it produces.
Using this technique the quasars become standard candles. Once the luminosity is known, the distance to the quasars can be calculated from the observed amount of radiation.
The researchers compiled UV data for 1,598 quasars to derive a relationship between UV and X-ray fluxes, and the distances to the quasars.
They then used this information to study the expansion rate of the Universe back to very early times, and found evidence that the amount of dark energy is growing with time.
“Since this is a new technique, we took extra steps to show that this method gives us reliable results,” said co-author Dr. Elisabeta Lusso, an astronomer at Durham University.
“We showed that results from our technique match up with those from supernova measurements over the last 9 billion years, giving us confidence that our results are reliable at even earlier times.”
Dr. Lusso and her colleague, Dr. Guido Risaliti from the University of Florence, also took great care in how their quasars were selected, to minimize statistical errors and to avoid systematic errors that might depend on the distance from Earth to the object.
If confirmed, this result would imply that dark energy is not the cosmological constant.
It could also help resolve an ongoing mismatch between the measurement of the Hubble constant — the rate of expansion of the Universe — based on local indicators and the measurement based on the CMB.
Using supernova observations, astronomers had previously reported that the Universe appears to be expanding faster now than was expected from its trajectory seen shortly after the Big Bang, when the CMB was produced.
“Some scientists suggested that new physics might be needed to explain this discrepancy, including the possibility that dark energy is growing in strength. Our new results agree with this suggestion,” Dr. Risaliti said.