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.
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Contacts
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Adam Riess
Space Telescope Science Institute/Johns Hopkins University, Baltimore, Maryland
410-338-6707
ariess@stsci.edu
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Adam Riess
Space Telescope Science Institute/Johns Hopkins University, Baltimore, Maryland
410-338-6707
ariess@stsci.edu
Source: HubbleSite
