Feb. 22, 2018
Donna Weaver / Ray Villard
Astronomers have used NASA's
Hubble Space Telescope to make the most precise measurements of the expansion
rate of the universe since it was first calculated nearly a century ago.
Intriguingly, the results are forcing astronomers to consider that they may be
seeing evidence of something unexpected at work in the universe.
That's because the latest
Hubble finding confirms a nagging discrepancy showing the universe to be
expanding faster now than was expected from its trajectory seen shortly after
the big bang. Researchers suggest that there may be new physics to explain the
inconsistency.
"The community is really
grappling with understanding the meaning of this discrepancy," said lead
researcher and Nobel Laureate Adam Riess of the Space Telescope Science Institute
(STScI) and Johns Hopkins University, both in Baltimore, Maryland.
Riess's team, which includes
Stefano Casertano, also of STScI and Johns Hopkins, has been using Hubble over
the past six years to refine the measurements of the distances to galaxies,
using their stars as milepost markers. Those measurements are used to calculate
how fast the universe expands with time, a value known as the Hubble constant.
The team’s new study extends the number of stars analyzed to distances up to 10
times farther into space than previous Hubble results.
But Riess's value reinforces
the disparity with the expected value derived from observations of the early
universe's expansion, 378,000 years after the big bang — the violent event that
created the universe roughly 13.8 billion years ago. Those measurements were
made by the European Space Agency's Planck satellite, which maps the cosmic
microwave background, a relic of the big bang. The difference between the two
values is about 9 percent. The new Hubble measurements help reduce the chance
that the discrepancy in the values is a coincidence to 1 in 5,000.
Planck's result predicted that
the Hubble constant value should now be 67 kilometers per second per megaparsec
(3.3 million light-years), and could be no higher than 69 kilometers per second
per megaparsec. This means that for every 3.3 million light-years farther away
a galaxy is from us, it is moving 67 kilometers per second faster. But Riess's
team measured a value of 73 kilometers per second per megaparsec, indicating
galaxies are moving at a faster rate than implied by observations of the early
universe.
The Hubble data are so precise
that astronomers cannot dismiss the gap between the two results as errors in
any single measurement or method. "Both results have been tested multiple
ways, so barring a series of unrelated mistakes," Riess explained,
"it is increasingly likely that this is not a bug but a feature of the
universe."
Explaining a Vexing
Discrepancy
Riess outlined a few possible
explanations for the mismatch, all related to the 95 percent of the universe
that is shrouded in darkness. One possibility is that dark energy, already
known to be accelerating the cosmos, may be shoving galaxies away from each
other with even greater — or growing — strength. This means that the
acceleration itself might not have a constant value in the universe but changes
over time in the universe. Riess shared a Nobel Prize for the 1998 discovery of
the accelerating universe.
Another idea is that the
universe contains a new subatomic particle that travels close to the speed of
light. Such speedy particles are collectively called "dark radiation"
and include previously-known particles like neutrinos, which are created in
nuclear reactions and radioactive decays. Unlike a normal neutrino, which
interacts by a subatomic force, this new particle would be affected only by
gravity and is dubbed a "sterile neutrino."
Yet another attractive
possibility is that dark matter (an invisible form of matter not made up of
protons, neutrons, and electrons) interacts more strongly with normal matter or
radiation than previously assumed.
Any of these scenarios would
change the contents of the early universe, leading to inconsistencies in
theoretical models. These inconsistencies would result in an incorrect value
for the Hubble constant, inferred from observations of the young cosmos. This
value would then be at odds with the number derived from the Hubble
observations.
Riess and his colleagues don't
have any answers yet to this vexing problem, but his team will continue to work
on fine-tuning the universe's expansion rate. So far, Riess's team, called the
Supernova H0 for the Equation of State (SH0ES), has decreased the uncertainty
to 2.3 percent. Before Hubble was launched in 1990, estimates of the Hubble
constant varied by a factor of two. One of Hubble's key goals was to help
astronomers reduce the value of this uncertainty to within an error of only 10
percent. Since 2005, the group has been on a quest to refine the accuracy of
the Hubble constant to a precision that allows for a better understanding of
the universe's behavior.
Building a Strong Distance
Ladder
The team has been successful
in refining the Hubble constant value by streamlining and strengthening the
construction of the cosmic distance ladder, which the astronomers use to
measure accurate distances to galaxies near to and far from Earth. The
researchers have compared those distances with the expansion of space as
measured by the stretching of light from receding galaxies. They then have used
the apparent outward velocity of galaxies at each distance to calculate the
Hubble constant.
But the Hubble constant's
value is only as precise as the accuracy of the measurements. Astronomers cannot
use a tape measure to gauge the distances between galaxies. Instead, they have
selected special classes of stars and supernovae as cosmic yardsticks or
milepost markers to precisely measure galactic distances.
Among the most reliable for
shorter distances are Cepheid variables, pulsating stars that brighten and dim
at rates that correspond to their intrinsic brightness. Their distances,
therefore, can be inferred by comparing their intrinsic brightness with their
apparent brightness as seen from Earth.
Astronomer Henrietta Leavitt
was the first to recognize the utility of Cepheid variables to gauge distances
in 1913. But the first step is to measure the distances to Cepheids independent
of their brightness, using a basic tool of geometry called parallax. Parallax
is the apparent shift of an object's position due to a change in an observer's
point of view. This technique was invented by the ancient Greeks who used it to
measure the distance from Earth to the Moon.
The latest Hubble result is
based on measurements of the parallax of eight newly analyzed Cepheids in our
Milky Way galaxy. These stars are about 10 times farther away than any studied
previously, residing between 6,000 light-years and 12,000 light-years from
Earth, making them more challenging to measure. They pulsate at longer
intervals, just like the Cepheids observed by Hubble in distant galaxies
containing another reliable yardstick, exploding stars called Type Ia
supernovae. This type of supernova flares with uniform brightness and is brilliant
enough to be seen from relatively farther away. Previous Hubble observations
studied 10 faster-blinking Cepheids located 300 light-years to 1,600
light-years from Earth.
Scanning the Stars
To measure parallax with
Hubble, the team had to gauge the apparent tiny wobble of the Cepheids due to
Earth's motion around the Sun. These wobbles are the size of just 1/100 of a
single pixel on the telescope's camera, which is roughly the apparent size of a
grain of sand seen 100 miles away.
Therefore, to ensure the
accuracy of the measurements, the astronomers developed a clever method that
was not envisioned when Hubble was launched. The researchers invented a
scanning technique in which the telescope measured a star's position a thousand
times a minute every six months for four years.
The team calibrated the true
brightness of the eight slowly pulsating stars and cross-correlated them with
their more distant blinking cousins to tighten the inaccuracies in their
distance ladder. The researchers then compared the brightness of the Cepheids
and supernovae in those galaxies with better confidence, so they could more
accurately measure the stars' true brightness, and therefore calculate
distances to hundreds of supernovae in far-flung galaxies with more precision.
Another advantage to this
study is that the team used the same instrument, Hubble's Wide Field Camera 3,
to calibrate the luminosities of both the nearby Cepheids and those in other
galaxies, eliminating the systematic errors that are almost unavoidably
introduced by comparing those measurements from different telescopes.
"Ordinarily, if every six
months you try to measure the change in position of one star relative to
another at these distances, you are limited by your ability to figure out
exactly where the star is," Casertano explained. Using the new technique,
Hubble slowly slews across a stellar target, and captures the image as a streak
of light. "This method allows for repeated opportunities to measure the
extremely tiny displacements due to parallax," Riess added. "You're
measuring the separation between two stars, not just in one place on the
camera, but over and over thousands of times, reducing the errors in
measurement."
The team's goal is to further
reduce the uncertainty by using data from Hubble and the European Space
Agency's Gaia space observatory, which will measure the positions and distances
of stars with unprecedented precision. "This precision is what it will
take to diagnose the cause of this discrepancy," Casertano said.
The team's results have been
accepted for publication by 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 conducts
Hubble science operations. STScI is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., in Washington, D.C.
For more about Hubble,
visit: www.nasa.gov/hubble
For additional imagery to this
story, visit: https://media.stsci.edu/news_release/news/2018-12
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Last Updated: Feb. 22,
2018
Editor: Karl Hille
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