The latest Hubble data lower the possibility that discrepancy is only a fluke to 1 in 100,000. This is a significant gain from a previous estimation, less than a year ago, of a chance of 1 in 3,000.
These most accurate Hubble measurements to date reinforce the idea that new physics may be needed to explain the mismatch.
"The Hubble tension between the early and late universe may be the most exciting development in cosmology in decades," said lead researcher and Nobel Laureate Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University in Baltimore, Maryland. "
Tightening the bolts on the '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 further and further away using their stars as milepost markers. Astronomers use these values, along with other measurements of the galaxies' light that reddens as it passes through a stretching universe, to calculate how fast the cosmos expands with time, a value known as the Hubble constant. Riess and his SH0ES (Supernovae H 0 for the Equation of State) team have been on a quest since 2005 to refine those 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 Large Magellanic Cloud. The observations helped astronomers "rebuild" the distance ladder by improving the comparison between those Cepheids and their more 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%
As the team's measurements became more accurate, their calculation of the Hubble constant has remained at odds with the expected value derived from observations of the early universe's expansion. These measurements were made by Planck, which maps the cosmic microwave background and relic afterglow from 380,000 years after the big bang.
The measurements have been thoroughly vetted, so astronomers can not currently dismiss the gap between the two results as due to a error in any single measurement or method.
"This is not just two experiments disagreeing," 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 on the measurement of how fast it should expand If these values do not agree, there is a very strong likelihood that we are missing something in the cosmological model that connects the two eras. "
How the new study was done
Astronomers have been using Cepheid variables as cosmic yardsticks to gauge nearby intergalactic distances for more than a century. But trying to harvest and a bunch of these stars was so time-consuming as to be almost unachievable. So, the team used a clever new method, called DASH (Drift And Shift), using Hubble as a "point-and-shoot" camera to snap quick images of extremely bright pulsing stars, eliminating the time-consuming need for precise pointing.
"When Hubble uses precise pointing by locking onto guide stars, it can only observe one Cepheid per 90-minute Hubble orbit around Earth, so it would be very costly for the telescope to observe each Cepheid," explained team member Stefano Casertano, also of STScI and Johns Hopkins. "Instead, we searched for groups of Cepheids close enough to each other that we could move between them without recalibrating the telescope pointing.This Cepheids are so bright, we only need to observe them for two seconds.This technique is allowing us to observe a "
The Hubble astronomers then combined their result with another set of observations, made by the Araucaria Project, a collaboration of the Cucheids for the duration of one orbit. between astronomers from Chile, the US, and Europe. This group made distance measurements to the Large Magellanic Cloud by observing the dimming of light as one star passes in front of its partner in eclipsing binary-star systems
The combined measurements helped the SHOES Team refine Cepheids' true brightness. With this more accurate result, the team could then "tighten the bolts" of the rest of the ladder that extends deeper into 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 away and the galaxy is from us, it seems to be 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 9% faster rate than the prediction of 67 kilometers (41.6 miles) per second per megaparsec, which comes from Planck's observations of the early universe, coupled with our present understanding of the universe  So, what could explain this discrepancy?
One explanation for the mismatch involves an unexpected appearance of dark energy in the young universe, which is now thought to contain 70% of the universe's contents. Proposed by astronomers at Johns Hopkins, the theory is dubbed "early dark energy," and suggests that the universe evolved as a three-act play.
Astronomers have already hypothesized that dark energy existed during the first seconds after the big bang and pushed matter through space, starting the initial expansion. Dark energy may also be the reason for the universe's accelerated expansion 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 account for the tension between the two Hubble constant values, Riess said.
Another idea is that the universe contains a new subatomic particle that travels near the speed of light. Such rapid particles are collectively called "dark radiation" and include previously known particles such as neutrinos, which are created in nuclear reactions and radioactive decays
Yet another attractive possibility is that dark matter (an invisible form of matter not made of protons , the neutrons, and electrons) interacts more strongly with the normal matter or radiation than previously assumed
But the true explanation is still a mystery
Riess does not have a response to this vexing problem, but his team will continue to use Hubble to reduce the uncertainties in the Hubble constant. Their goal is to reduce the uncertainty to 1%, which should help astronomers identify the cause of the discrepancy.
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, is conducting Hubble Science Operations. STScI is operated for NASA by the Association of Universities for Astronomy Research in Washington, D.C.