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Astronomers using NASA's Hubble Space Telescope said they have crossed a significant threshold by revealing a gap between the two key techniques for measuring the rate of universe expansion. The recent study confirms that new theories may be needed to explain the forces that shaped the cosmos.
A brief recap: The universe is growing every second. The space between the galaxies expands, like a baked dough. But how fast is the universe expanding? While Hubble and other telescopes are trying to answer this question, they have come up against an intriguing difference between what scientists predict and what they observe.
Hubble's measurements suggest a rate of expansion faster than expected in the modern universe, depending on how the universe appeared more than 13 billion years ago. These measurements of the first universe come from the Planck satellite of the European Space Agency. This discrepancy has been identified in scientific papers in recent years, but it is not known if the differences in measurement techniques are to blame, or if the difference could be the result of unfortunate measures.
The latest Hubble data reduces the risk that the gap is only a stroke of luck for 1 in 100,000. This is a significant gain compared to an earlier estimate it less than a year ago, with a probability of 1 in 3,000.
The most accurate Hubble measurements to date support the idea that new physics may be needed to explain the mismatch.
"Hubble's tension between the first and the last universe is perhaps the most exciting development of cosmology in decades," said Adam Riess, Senior Scientist and Nobel Prize winner of the Space Telescope Science Institute (STScI) and Johns Hopkins University, Baltimore, Maryland. "This imbalance is growing and has now reached a point that it is really impossible to consider as a stroke of luck.This disparity probably could not happen simply by chance."
Tighten the bolts on "cosmic distance scale"
Scientists use a "cosmic distance scale" to determine the distance that separates objects from the universe. This method depends on the precise measurement of distances from neighboring galaxies, and their displacement into increasingly distant galaxies, using their stars as mile markers. Astronomers use these values, as well as other light measurements of galaxies that blush at the passage of a stretching universe, to calculate the speed with which the cosmos expands over time, a value known as Hubble constant. Riess and his SHOES (Supernovae H0 for the state equation) have been seeking since 2005 to refine these distance measurements with Hubble and to refine the Hubble constant.
In this new study, astronomers used Hubble to observe 70 pulsating stars called Cepheid variables in the Great Magellanic Cloud. The observations helped the astronomers to "reconstruct" the distance scale by improving the comparison between these Cepheids and their more distant cousins in the supernova galactic hosts. The Riess team reduced Hubble's constant value uncertainty to 1.9%, compared to an earlier estimate of 2.2%.
As the team's measurements became more accurate, their calculation of Hubble's constant remained at odds with the expected value derived from observations of the expansion of the early universe. These measurements were made by Planck, who maps the cosmic background of microwaves, a relic of 380,000 years after the big bang.
The measurements have been thoroughly checked, so that astronomers currently can not consider the difference between the two results as being due to an error in a single measurement or method. Both values were tested in several ways.
"It's not just two experiences that disagree," said Riess. "We measure something fundamentally different, one is a measure of the speed at which the universe is developing today, as we see it.The other is a prediction based on the physics of the first universe and measures of the speed at which it should develop If these values do not match, it becomes very likely that we are missing something in the cosmological model that connects the two epochs. "
How the new study was made
Astronomers use Cepheid variables as cosmic measurement criteria to evaluate nearby intergalactic distances for more than a century. But trying to harvest some of these stars was taking so much time that it was almost unachievable. The team used a new intelligent method, called Drift And Shift (DASH), using Hubble as a "viewfinder and finder" camera to capture fast images of extremely bright pulsating stars, eliminating the tedious need for finger pointing. .
"When Hubble uses accurate pointing by locking himself on the guide stars, he can only observe one Cepheid per 90-minute orbit around the Earth, so it would be very expensive for the telescope to observe each Cepheide, "said Stefano Casertano, a member of the team. also from STScI and Johns Hopkins. "Instead, we have been looking for groups of Cepheids close enough to move around without recalibrating the telescope pointer.These Cepheids are so bright that we only need to observe them for two seconds This technique allows us to observe a dozen Cepheids during an orbit, so we stay under control of the gyro and keep DASHING very quickly. "
Hubble's astronomers then combined their findings with another set of observations, made by the Araucaria Project, a collaboration between astronomers from institutions in Chile, the United States and Europe. This group made distance measurements up to the great magellanic cloud by observing the decrease in light as a star passes in front of its partner, which eclipses binary star systems.
The combined measurements allowed the SH0ES team to refine the true brightness of the Cepheids. With this result more accurate, the team could then "tighten the bolts" from the rest of the distance scale that extends deeper into the space.
The new estimate of the Hubble constant is 74 kilometers per second per megaparsec. That means that, about 3.3 million light years away, a galaxy is far away from us, it seems to move 74 km faster per second, due to the expansion of the galaxy. universe. The number indicates that the universe is growing at a rate 9% faster than the prediction of 67 kilometers (41.6 miles) per second per megaparsec, which results from Planck's observations on the primitive universe, associated with to our current understanding of the universe.
So, what could explain this difference?
One of the explanations for this inadequacy is the unexpected appearance of a dark energy in the young universe, which now represents 70% of the content of the universe. Offered by Johns Hopkins astronomers, the theory is dubbed "early dark energy" and suggests that the universe has evolved as a play in three acts.
Astronomers have already hypothesized that dark energy existed for the first few seconds after the big bang and pushed matter into space from the moment it developed. Dark energy can also be the reason for the accelerated expansion of the universe today. The new theory suggests that there was a third episode of black energy shortly after the big bang, which expanded the universe faster than predicted by astronomers. The existence of this "early dark energy" could explain the tension between Hubble's two constant values, Riess said.
Another idea is that the universe contains a new subatomic particle that moves at a speed close to that of light. These fast particles are collectively called "dark radiation" and include previously known particles, such as neutrinos, which are created during nuclear reactions and radioactive decays.
Another interesting possibility is that dark matter (an invisible form of matter not composed of protons, neutrons and electrons) interacts more strongly than expected with normal matter or radiation.
But the true explanation remains a mystery.
Riess has no answer to this thorny problem, but his team will continue to use Hubble to reduce the uncertainties of Hubble's constant. Their goal is to reduce uncertainty to 1%, which should help astronomers identify the cause of the discrepancy.
The results of the team have been accepted for publication in The Astrophysical Journal.
The Hubble Space Telescope is an international cooperation project between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, operates the telescope. The Institute of Space Telescope Sciences (STScI) in Baltimore, Maryland, conducts Hubble's scientific activities. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
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