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It is a terrestrial view of the Earth telescope on the Great Magellanic Cloud, a satellite galaxy of our Milky Way. The inlaid image, taken by the Hubble Space Telescope, reveals one of the many star clusters scattered throughout the dwarf galaxy. Members of the group include a special class of pulsating stars called the Cepheid variable, which clears and darkens at a predictable rate corresponding to its intrinsic luminosity. Once astronomers have determined this value, they can measure the light of these stars to calculate a precise distance to the galaxy. When Hubble's new observations were correlated with a distance measurement technique independent of the large magellanic cloud (using direct trigonometry), researchers were able to strengthen the foundations of the so-called "cosmic distance scale". the accuracy of the speed of expansion of the universe, called Hubble constant, has greatly improved the accuracy. Credits: NASA, ESA, A. Riess (STScI / JHU) and Palomar Digitized Sky Survey
Astronomers using NASA's Hubble Space Telescope said they have crossed a significant threshold by revealing a mismatch between the two key techniques for measuring the rate of expansion of the universe. 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 mismatch is growing and has now reached a point that is really impossible to consider as a stroke of luck. This disparity probably could not happen by chance. "
Tighten bolts on "cosmic distance scale"
Scientists use a "cosmic distance scale" to determine the distance between objects and 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, along with other measurements of the light of galaxies that redden as it passes through a stretching universe, to calculate the speed with which the cosmos expands over time, a value known as Hubble constant. Riess and his SH0ES team (Supernovae H0 for the state equation) have been looking since 2005 to refine these distance measurements with Hubble and refine Hubble's 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 the uncertainty of the constant value of Hubble to 1.9%, compared to an earlier estimate of 2.2%.
As the team's measurements became more accurate, their calculation of the Hubble constant remained at odds with the expected value derived from observations of early universe expansion. 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 expanding today, as we see it. The other is a prediction based on the physics of the first universe and on measures of the speed at which it should develop. If these values do not match, it becomes very likely that we miss something in the cosmological model that connects the two eras. "
This illustration shows the three basic steps used by astronomers to calculate the speed with which the universe expands in time, a value called Hubble's constant. All the steps are to build a strong "cosmic distance scale", starting by measuring precise distances from neighboring galaxies, then moving further and further. This "scale" is a series of measurements of different types of astronomical objects with intrinsic brightness that researchers can use to calculate distances. Among the most reliable for shorter distances are the Cepheid variables, stars that pulsate at predictable speeds that indicate their intrinsic brightness. Astronomers have recently used the Hubble Space Telescope to observe 70 Cepheid variables in the nearby Great Magellan Cloud, in order to measure their distance from this galaxy with the least distance. Astronomers compare the measurements of nearby Cepheids to those of more distant galaxies, which also include another cosmic criterion, explosive stars called Type Ia supernovae. These supernovas are much brighter than the Cepheid variables. Astronomers use them as "kilometric beacons" to measure the distance between Earth and distant galaxies. Each of these markers is based on the previous step of "scale". By extending the scale with the help of different types of reliable mile markers, astronomers can reach very large distances in the universe. Astronomers compare these distance values with measurements of the light of a whole galaxy, which blushes more and more with distance, because of the uniform expansion of space. Astronomers can then calculate the speed of expansion of the cosmos: the Hubble constant. Credits: NASA, ESA and A. Feild (STScI)
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 then used a new intelligent method, called DASH (Drift And Shift), using Hubble as a "aim and shoot" camera to capture fast images of extremely bright pulsating stars, eliminating the need for time to show finger.
"When Hubble uses accurate pointing by locking on the guide stars, he can only observe one Cepheid per 90-minute orbit around the Earth." It would therefore be very expensive for the telescope to observe each Cepheid, "said Stefano Casertano, member of the team, also from STScI and Johns Hopkins. "Instead, we searched for groups of Cepheids close enough to each other to move between them without having to recalibrate the telescope's pointing. These Cepheids are so bright that you just have to watch them for two seconds. This technique allows us to observe a dozen Cepheids during an orbit. We remain on the control of the gyroscope and continue to "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. This figure indicates that the universe is growing 9% faster than the prediction of 67 kilometers per second per megaparsec, which results from Planck's observations of the primitive universe and our current understanding of the universe.
So, what could explain this difference?
One explanation for this inadequacy is the unexpected appearance of dark energy in the young universe, which now accounts for 70% of the universe's content. Offered by Johns Hopkins' astronomers, the theory is dubbed "Dark Energy Early" 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 explain the acceleration of the expansion of the universe. 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 that exists 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 in 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 the Hubble 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 the Maryland Greenbelt manages 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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