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An important threshold crossed by the mystery of the expansion coefficient of the Universe

The secret of the universe expansion rate is falling

It is a ground-based telescope view of the Great Magellanic Cloud, our Milky Way satellite galaxy. The picture taken by Hubble Space Telescope shows that one of the many star clusters is scattered around the dwarf galaxy. Cluster members include a special class of pulsating stars, called the Cepheid variable, which is bright and flashing at a predictable rate that matches its internal brightness. When astronomers determine this value, they can measure the light from these stars to calculate the exact distance to the galaxy. When the new Hubble observations are linked to an independent distance measurement method for the Great Magellanic Cloud (using simple trigonometry), researchers were able to reinforce the so-called "cosmic distance ladder" foundation. greatly improved the accuracy of the speed with which the universe expands, called the Hubble constant. Credits: NASA, ESA, Riess (STScI / JHU) and Palomar Digitized Sky Survey

Astronomers who use the NASA Hubble Space Telescope say they have crossed an important threshold, revealing the difference between the two main methods to measure the universe's expansion rate. A recent study reinforces the case when new theories may be needed to explain the forces that make up space.

Brief: The universe gets bigger every second. The space between the galaxies extends, for example, the dough rises in the oven. But how fast is the universe growing? Since Hubble and other telescopes are trying to answer this question, they face an intriguing difference between what scientists predict and what they observe.

Hubble's measurements show a faster pace of expansion in today's universe than expected based on how the universe appeared over 13 billion years ago. These early measurements of the universe come from the satellite of the European Space Agency Planck. This discrepancy has been found in scientific articles over the last several years, but it is unclear whether the fault is different in the measurement methods, or whether the difference may result from failed measurements.

The latest Hubble data downgrades the probability of a discrepancy of only 1 in 100,000. This is a significant benefit from an earlier estimate of less than a year ago, with the possibility of 1 in 3,000.

The most accurate Hubble measurements so far confirm the idea that new physics may be needed to explain the discrepancy.

"Hubble's tension between the early and late universes can be the most exciting development of cosmology in the decades," said a leading researcher and Nobel laureate Adam Riess of the Space Telescope Science Institute (STScI) and John Hopkins University, Baltimore, Maryland. “This discrepancy has increased and has now reached a point that is really impossible to reject as a fluke. This difference could not have been credible only by chance. ”

Tightening the screws for the cosmic distance ladder

Scientists use the "cosmic distance ladder" to determine how far everything is in the universe. This method depends on measuring the exact distances to the nearby galaxies and further on the galaxies further and further using their stars as milepost markers. Astronomers use these values ​​along with other galaxy light measurements that decrease when it passes through the stretching universe to calculate how quickly space expands with time, a value known as the Hubble constant. Riess and his SH0ES (Supernovae H0 national equation) team have been searching since 2005 to clarify these distance measurements with Hubble and to refine the Hubble constant.

In this new study, astronomers used Hubble to observe 70 throbbing stars called Cepheid variables in the large Magellanic Cloud. Observations helped astronomers 'rebuild' the remote ladder, improving the comparison between these Cepheids and their cousins, the owners of the supernova galaxy. Riess's team reduced Hubble's constant value uncertainty to 1.9% from its previous 2.2% estimate.

Because team measurements have become more accurate, calculating them for Hubble has been at odds with the expected value obtained by observing the expansion of the early universe. These measurements were made by Planck, which marks the cosmic microwave background, the relic of the relic from 380,000 years after the big bang.

Measurements have been thoroughly tested, so astronomers cannot now eliminate the difference between both results as a mistake in a single measurement or method. Both values ​​have been tested in several ways.

"It's not just two experiments that don't agree," Riess explained. “We measure something completely different. One of them is the measurement of how fast the universe is expanding today, as we see it. The second is a prediction based on the physics of the early universe and on how quickly it should expand. If you disagree with these values, it is very likely that we will lose something in the cosmological model that connects both ages. ”

The Universe expands its speed with new Hubble data

This illustration shows three main steps for astronomers to calculate how quickly the universe expands over time and is called Hubble Constant. All the steps are related to the creation of a strong "cosmic distance staircase", starting with precise measurements of distances to nearby galaxies and then on to the galaxies further and further. This “ladder” is a series of measurements of astronomical objects with internal brightness that researchers can use to calculate distances. One of the most reliable shorter distances is Cepheid variables, stars that pulse at predictable rates that indicate their internal brightness. Astronomers have recently used the Hubble Space Telescope to observe 70 Cepheid variables in the nearby Great Magellanic Cloud to perform the most accurate measurement of this galaxy distance. Astronomers compare the measurements of nearby Cepheids with those of the galaxies that are further on, including another cosmic measurement, an exploding star, called the Ia supernova. These supernovae are much brighter than Cepheid variables. Astronomers use them as "milepost markers" to measure the distance from Earth to the ultra-expanded galaxies. Each of these markers is based on the previous step of the staircase. By expanding the stairs using a variety of reliable mileposta markers, astronomers can reach long distances in general. Astronomers compare these distance values ​​with the measurements of the whole galaxy, which is increasingly being reduced by distance, thanks to a unified expansion of space. Then astronomers can calculate how fast space expands: Hubble's constant. Credits: NASA, ESA and Feild (STScI)

How was the new study done?

Astronomers have used Cepheid variables as cosmic criteria to estimate the closest intergalactic distances for more than a century. But the attempt to remove a bunch of these stars was so time consuming to be almost unattainable. So, the team used a smart new method called DASH (Drift And Shift) using Hubble as a point-and-shoot camera to quickly display very bright pulsating stars that eliminate the time-consuming need for accurate pointing.

“If Hubble's exact reference is fixed to the guidelines, it can only observe one Cepheid for every 90-minute Hubble orbit around the Earth. So it would be very expensive for a telescope to observe every Cepheid, ”explained Stefan Casertano, a member of STScI and Johns Hopkins. “Instead, we were looking for groups of cepheids that were close enough to each other to move between them without recalibrating the telescope pointer. These cepheids are so bright, we just need to follow two seconds. This method allows us to observe twelve cepheids on one orbit. So we remain in control of the gyroscope and keep DASHing very fast. ”

Hubble's astronomers then combined their results with another set of observations made by the Araucaria project, collaboration between astronomers from institutions in Chile, the United States and Europe. This group made distance measurements for the large Magellanic cloud, observing the darkening of the light when one star goes over the front of his partner to close the binary star systems.

The combined measurements helped the SH0ES team clarify the true brightness of Cepheids. With this more accurate result, the team was able to "tighten" the other distance stair bolts that deepen into space.

The new estimate for Hubble Constant is 74 kilometers (46 miles) per second per megaparre. This means that every 3.3 million light years away from the galaxy is from us, and that seems to be moving 74 kilometers (46 miles) as a result of the universe movement. The number indicates that the universe is expanding by 9% faster than the forecast of 67 kilometers per second on the megaparses coming from Planck's observations of the early universe, along with our current understanding of the universe.

So who could explain this discrepancy?

One of the explanations for the mismatch is the unexpected appearance of dark energy in the new universe, which is believed to now account for 70% of the universe. John Hopkins' astronomers suggested that theory is called "early dark energy" and suggests that the universe developed as a three-point game.

Astronomers have already assumed that the dark energy existed in the first seconds after the big bang and pushed the entire space into the initial expansion. Dark energy can also be a reason for today's rapid expansion. The new theory shows that the third dark energy episode was not long after the big bang that expanded the universe faster than the astronomers had predicted. The existence of this "early dark energy" could create tension between the two Hubble constant values, said Riess.

Another idea is that the universe has a new subatomic particle that travels close to the speed of light. Such rapid particles are referred to as "dark radiation" and include pre-known particles such as neutrinos created during nuclear reactions and during radioactive damage.

Another attractive option is that the dark substance (an invisible type of material that is not made of protons, neutrons and electrons) interacts more strongly with a normal substance or radiation than previously accepted.

But the real explanation is still a mystery.

Riessam has no answer to this unpleasant problem, but his team will continue to use Hubble to reduce the uncertainty of the Hubble constant. Their goal is to reduce the uncertainty to 1%, which would help astronomers identify the cause of non-compliance.

Team results are accepted for publication in the Astrophysics magazine.

Hubble Space Telescope is an international cooperation project between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center Greenbelt, Maryland manages a telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, is engaged in Hubble science activities. STScI NASA is chaired by the Association of Astronomical Research Universities in Washington, DC.

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