Important cross threshold in the rate of expansion of the mystery of the universe

The expansion rate of the mystery of the universe expands

This is a view from the terrestrial telescope of the Large Magellanic Cloud, a satellite galaxy of our Milky Way. The box image, taken by the Hubble Space Telescope, reveals one of the many star clusters scattered throughout the dwarf galaxy. The members of the group include a special clbad of pulsating star called Cepheid variable, which is illuminated and attenuated at a predictable speed corresponding to its intrinsic brightness. Once astronomers determine that value, they can measure the light from these stars to calculate an accurate distance to the galaxy. When Hubble's new observations correlate with an independent distance measurement technique to the Large Magellanic Cloud (using direct trigonometry), the researchers were able to strengthen the base of the so-called "cosmic distance staircase". This "fine-tuning" significantly improved the accuracy of the speed at which the universe is expanding, called the Hubble constant. Credits: NASA, ESA, A. Riess (STScI / JHU) and Palomar Digitized Sky Survey

Astronomers using NASA's Hubble Space Telescope say they have crossed an important threshold by revealing a discrepancy between the two key techniques for measuring the expansion rate of the universe. The recent study reinforces the case that new theories may be necessary to explain the forces that have shaped the cosmos.

A brief summary: The universe is getting bigger every second. The space between the galaxies stretches, like the mbad that rises in the oven. But how fast is the universe expanding? When Hubble and other telescopes try to answer this question, they have found an intriguing difference between what scientists predict and what they observe.

Hubble measurements suggest a faster rate of expansion in the modern universe than expected, based on how the universe appeared more than 13 billion years ago. These measurements of the primitive universe come from the Planck satellite of the European Space Agency. This discrepancy has been identified in scientific articles in recent years, but it is not clear if differences in measurement techniques are to blame, or if the difference could be due to unfortunate measurements.

The latest Hubble data reduces the possibility that the discrepancy is just a chance of 1 in 100,000. This is a significant gain from a previous estimate, less than a year ago, of a possibility of 1 in 3,000.

These more accurate Hubble measurements to date reinforce the idea that new physicists may be needed to explain the mismatch.

"Hubble's tension between the early universe and the later universe may be the most exciting development in cosmology in decades," said principal investigator and Nobel laureate Adam Riess of the Space Telescope Science Institute (STScI, for its acronym in English) and Johns Hopkins University in Baltimore, Maryland. "This mismatch has been growing and now has reached a point that is really impossible to dismiss as a coincidence. This disparity could not occur plausibly just by chance. "

Tightening the screws in the 'cosmic distance staircase'

Scientists use a "cosmic distance staircase" to determine how far things are in the universe. This method depends on making accurate measurements of distances to nearby galaxies and then moving to galaxies farther and farther, using their stars as landmark markers. Astronomers use these values, along with other measurements of light from galaxies that redden as they pbad through a universe of stretching, to calculate how fast the cosmos expands with time, a value known as the constant of Hubble. Riess and his team SH0ES (Supernovae H0 for the equation of state) have been in a search since 2005 to refine those distance measurements with Hubble and adjust the Hubble constant.

In this new study, astronomers used Hubble to observe 70 pulsating stars called Cepheid variables in the Large Magellanic Cloud. The observations helped the astronomers to "reconstruct" the distance scale by improving the comparison between the Cepheids and their more distant cousins ​​in the galactic supernova hosts. The Riess team reduced the uncertainty in its Hubble constant value to 1.9% from a previous estimate of 2.2%.

As the measurements of the equipment have become more accurate, its calculation of the Hubble constant has remained at odds with the expected value derived from observations of the early universe expansion. Those measurements were made by Planck, who maps the cosmic microwave background, a relic glow 380,000 years after the Big Bang.

The measurements have been thoroughly examined, so astronomers can not rule out the gap between the two results due to an error in any measurement or method. Both values ​​have been tested in multiple ways.

"This is not just two experiments in disagreement," Riess explained. "We are measuring something fundamentally different. One is a measure of how fast the universe is expanding today, as we see it. The other is a prediction based on the physics of the early universe and the measurements of how fast it should expand. If these values ​​do not match, there is a strong likelihood that something is missing in the cosmological model that connects the two eras. "

The expansion rate of the universe is extended with the new Hubble data

This illustration shows the three basic steps that astronomers use to calculate how fast the universe expands over time, a value called the Hubble constant. All steps involve building a strong "cosmic distance staircase," beginning with measuring precise distances to nearby galaxies and then moving to galaxies farther and farther. This "ladder" is a series of measurements of different types of astronomical objects with an intrinsic brightness that researchers can use to calculate distances. Among the most reliable for shorter distances are the Cepheid variables, the stars that pulsate at predictable rates that indicate their intrinsic brightness. Astronomers recently used the Hubble Space Telescope to observe 70 Cepheid variables in the nearby Large Magellanic Cloud to measure the most accurate distance to that galaxy. Astronomers compare the measurements of nearby Cepheids with those of farther galaxies that also include another cosmic rod, the explosive stars called Type Ia supernovas. These supernovas are much brighter than the Cepheid variables. Astronomers use them as "milestone markers" to measure the distance from Earth to remote galaxies. Each of these markers is based on the previous step in the "staircase". By extending the ladder using different types of reliable markers, astronomers can reach very large distances in the universe. Astronomers compare these distance values ​​with measurements of the light of an entire galaxy, which reddens more and more with distance, due to the uniform expansion of space. Astronomers can calculate how fast the cosmos is expanding: the Hubble constant. Credits: NASA, ESA and A. Feild (STScI)

How the new study was conducted

Astronomers have been using Cepheid variables as cosmic criteria to measure nearby intergalactic distances for more than a century. But trying to harvest a handful of these stars was so slow as to be almost unattainable. Therefore, the team employed a new and intelligent method, called DASH (Drift And Shift), which uses Hubble as a "point-and-shoot" camera to capture fast images of extremely bright pulsating stars, eliminating the need of precision time. pointing

"When Hubble uses precise targeting by blocking itself in the guide stars, it can only observe one Cepheid for each 90-minute Hubble orbit around the Earth. Therefore, it would be very expensive for the telescope to observe each Cepheid, "said team member Stefano Casertano, also of STScI and Johns Hopkins." Instead, we looked for groups of Cepheids close enough together so that we could move between them. without recalibrating the telescope pointing in. 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.Therefore, we keep under gyroscope control and follow "DASHING" very fast "

The Hubble astronomers then combined their result with another set of observations, made by the Araucaria Project, a collaboration among astronomers from institutions in Chile, the USA. UU And Europe. This group made distance measurements to the Large Magellanic Cloud by observing the attenuation of light as a star pbades in front of its companion in eclipsing binary star systems.

The combined measures helped the SH0ES team refine the true brightness of the Cepheids. With this more accurate result, the team could "tighten the bolts" of the rest of the ladder away that extends deeper into the space.

The new estimate of the Hubble constant is 74 kilometers (46 miles) per second per megaparsec. This means that for every 3.3 million light years farther, a galaxy is from us, it seems that it is moving 74 kilometers (46 miles) per second faster, as a result of the expansion of the universe. The number indicates that the universe is expanding at a rate of 9% faster than the prediction of 67 kilometers (41.6 miles) per second per megaparsec, which comes from Planck's observations of the early universe, along with our current understanding of the universe .

So, what could explain this discrepancy?

An explanation for the mismatch implies an unexpected appearance of dark energy in the young universe, which is believed to now comprise 70% of the contents of the universe. Proposed by astronomers at Johns Hopkins, the theory is called "early dark energy" and suggests that the universe evolved as a work of three acts.

Astronomers have already hypothesized that dark energy existed during the first seconds after the Big Bang and pushed matter across space, beginning the initial expansion. Dark energy can also be the reason for the accelerated expansion of the current universe. The new theory suggests that there was a third episode of dark energy shortly after the Big Bang, which expanded the universe faster than astronomers had predicted. The existence of this "early dark energy" could explain the tension between the two constant Hubble values, Riess said.

Another idea is that the universe contains a new subatomic particle that travels close to the speed of light. Such fast particles are collectively referred to as "dark radiation" and include previously known particles such as neutrinos, which are created in nuclear reactions and radioactive decays.

Another attractive possibility is that dark matter (an invisible form of matter that is not made up of protons, neutrons and electrons) interacts more strongly with normal matter or radiation than previously badumed.

But the real explanation remains a mystery.

Riess does not have an answer to this annoying problem, but his team will continue using Hubble to reduce the uncertainties in the Hubble constant. Its goal is to reduce uncertainty to 1%, which should help astronomers identify the cause of the discrepancy.

The team's results 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). The NASA Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Science Institute of the Space Telescope (STScI) in Baltimore, Maryland, conducts the scientific operations of Hubble. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

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