Astronomers using NASA's Hubble Space Telescope say they have crossed an important threshold to reveal a discrepancy between the two key measurement techniques of the expansion rate of the universe. The recent study strengthens the case that new theories may be needed to explain the forces that have formed the cosmos.
A brief review: The universe grows bigger every second. The space between galaxies extends as the dough rises in the oven. But how fast is the universe expanding? As Hubble and other telescopes seek to answer this question, they have entered into an exciting difference between what scientists predict and what they observe.
Hubble measurements indicate a faster rate of expansion in the modern universe than expected, based on how the universe appeared more than 1
The latest Hubble data reduces the possibility that inconsistency is only a fluke to 1 in 100,000. This is a significant gain from a previous estimate, less than a year ago, with a chance of 1 in 3,000.
These most accurate Hubble measurements so far reinforce the idea that new physics may be needed to explain the error parameter.  "The Hubble tension between the early and late universes may be the most exciting development in cosmology for decades," said senior researcher and Nobel laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University in Baltimore, Maryland. "This mismatch has grown and has now reached a point that is really impossible to reject as a fluke. This difference was unlikely to occur just randomly."
Tightening bolts on cosmic distance ladder
Scientists use a "cosmic distance ladder" to determine how far away 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 away using their stars as milestone markers. The astronomers use these values along with other measurements of the light of the galaxies, which roots as it passes through a stretch universe, to calculate how quickly the cosmos expands with time, a value known as the Hubble constant. Riess and his SH0ES (Supernovae H0 for equality) have since 2005 sought to refine these distance measurements with Hubble and fine-tune 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 astronomers "rebuild" the distance ladder by improving the comparison between the Cepheids and their distant cousins in the galactic hosts of supernovae. Riess's team reduced the uncertainty in their Hubble constant value to 1.9% from a previous estimate of 2.2%.
Since the team's measurements have become more accurate, their Hubble constant calculation has remained unlike the expected value derived from observations of the early universe expansion. These measurements were made by Planck, which maps the cosmic microwave background, a relic afterglow from 380,000 years after the big bang.
The measurements have been thoroughly investigated so that astronomers cannot currently reject the gap between the two results as a result of a single measurement or method error. Both values have been tested in several ways.
"This is not just two experiments disagreeing," explained Riess. "We measure something fundamentally different. One is a measure of how fast the universe is expanding today as we see it. The second is a prediction based on the early universe's physics and on measurements of how quickly it should expand. do not agree, there is a very strong likelihood that we are missing something in the cosmological model that connects the two epochs. "
How the new study was conducted
Astronomers have used Cepheid variables as a cosmic scale to measure nearby intergalactic distances for more than a century . But trying to harvest a bunch of these stars was so time-consuming to be almost unmanageable. Then the team has used a smart new method called DASH (Drift And Shift), using Hubble as a "point-and-shoot" camera to get fast pictures of the extremely bright pulsating stars, eliminating the time-consuming need for precise points.
"When Hubble uses precise pointing by locking on steering stars, it can only observe a Cepheid per every 90-minute Hubble circuit around the Earth. So it would be very expensive for the telescope to observe every Cepheid," team member explained. Stefano Casertano, also by STScI and Johns Hopkins. "Instead, we searched for groups of Cepheids close enough to each other, that we could move between them without calibrating the telescope again. These Cepheids are so bright we only need to observe them for two seconds. This technique allows us to observe a dozen Cepheids during one lap. So we remain on the gyroscope control and hold "DASHing" around very quickly. "The Hubble astronomers then combined their results 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 for Large Magellanic Cloud by observing the dimming as a star passes in front of its partner in the eclipse of binary star systems.
The combined measurements helped SH0ES Team to sense Cepheid's true brightness. With this more precise result, the team could then "tighten the bolts" of the rest of the distance ladder extending deeper into the room.
The new estimate of the Hubble constant is 74 kilometers (46 miles) per. Second per second Megaparsek. This means that for every 3.3 million light-years further away is a galaxy from us, it seems to be moving 74 kilometers (46 miles) per second faster due to the expansion of the universe. The figure indicates that the universe is expanding at a 9% faster rate than the 67-kilometer (41.6 miles) per second per megaparsec prediction that comes from Planck's observations of the early universe, combined with our current understanding of the universe.  So, what could explain this disagreement?
An explanation of the mismatch involves an unexpected appearance of dark energy in the young universe, which is believed to contain 70% of the contents of the universe. Suggested by astronomers at Johns Hopkins, the theory is called "early dark energy" and suggests that the universe evolved as a three-act play.
Astronomers have already suggested that dark energy existed during the first few seconds after the big bang and pushed material throughout the room, beginning with the first expansion. Dark energy can also be the cause of the accelerated expansion of the universe today. The new theory suggests that there was a third dark-energy episode not long after the big bang, which expanded the universe faster than astronomers had predicted. The existence of this "early dark energy" could draw the tension between the two Hubble constant values, Riess said.
Another idea is that the universe contains a new subatomic particle that moves close to the speed of light. Such fast particles are collectively called "dark radiation" and include previously known particles such as neutrinos created in nuclear reactions and radioactive decays.
Yet another attractive option is that dark matter (an invisible form of matter that does not consist of protons, neutrons and electrons) interacts more strongly with normal matter or radiation than previously thought.
But the true explanation is still a mystery.
Riess does not answer this worrying problem, but his team will continue to use Hubble to reduce the uncertainty of the Hubble constant. Their goal is to reduce uncertainty to 1%, helping astronomers identify the cause of the disagreement.
The team's results have been accepted for publication in The Astrophysical Journal.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, performs Hubble Science operations. STScI is operated by NASA by the Association of Universities for Research in Astronomy in Washington, D.C.