Perhaps resolving or exacerbating a
longstanding dilemma in cosmology, the most precise measurement to date of
faraway stars that undergo periodic changes in brightness may prompt a
reconsideration of the rate at which the universe expands. According to a new study, this result lends
credence to the discrepancy between the two leading techniques for determining
the rate at which the universe is expanding.
The Cepheid variable stars studied by the
Stellar Standard Candles and Distances team using data from Europe’s Gaia spacecraft are reliable standards for determining the vastness of the
universe because of their regular pulsations. Methods like using observations
of Type 1asupernovae serve as a foundation for the Cepheid star measurement approach. Supernovas, which are massive explosions at
the end of the lives of huge stars, produce such a consistent amount of light
that they are often referred to as “standard candles” and play a
significant role in what astronomers refer to as the “cosmic distance
ladder.” This new study reinforces the Cepheid star distance measurement
method, which adds another “rung” to that figurative ladder.
As the saying goes, “If you can’t beat
’em, join ’em.” So, if you’ve got the chops to hang with the big dogs, then
you’ve got the brains to outsmart ’em. “Thanks to this trick, we were able to
take advantage of the best knowledge of Gaia’s parallax measurements while also
benefiting from the gain in precision provided by the many cluster member
stars,” Anderson added. This has allowed us to maximize the precision of
Gaia parallaxes and gives the most stable rung on which to build the distance
ladder.
The Hubble constant, the rate at which the
universe is expanding, can alternatively be calculated using the cosmic distance
ladder. The “Hubble tension,” a concern with the rate at which the
universe is expanding, is exacerbated by the latest recalibration of the
Cepheid “rung.” A cosmic distance ladder is a tool used by
astronomers to calculate the rate at which the universe is expanding. The stars
and other objects within the galaxies with known distances form a symbolic
“ladder” in this illustration. Scientists can determine the expansion
rate by combining these distance measurements with the velocities at which
objects are receding from us. (NASA/JPL-Caltech is credited for the image)
What is the Hubble tension?
The discovery by Edwin Hubble that the
universe is expanding, rather than remaining static, sent shockwaves across the
fields of physics and astronomy in the early 20th century. The name
“Hubble constant” was coined to refer to this rate of growth. In the late 1990s, astronomers observed
distant supernovae and changed this idea forever by showing that the expansion
of the universe is, in fact, speeding. Since then, astronomers and cosmologists
have had a difficult time agreeing on how to measure the Hubble constant
because there are two primary methods for doing so, and they provide different
results.
One technique analyzes the velocities of
galaxies as a function of distance to derive a Hubble constant of about 73 1
km/s/Mpc, where 1 Megaparsec is roughly 3.26 million light-years away. This is
the “late time” answer, so called since it is based on very recent
cosmological data. Light from an event shortly after the Big
Bang termed “the last scattering,” in which electrons mixed with
protons to form the first atoms, can also be used to measure the Hubble
constant. The light was instantly unimpeded in its ability to traverse the cosmos,
as unbound electrons had previously scattered photons (particles of light)
substantially, preventing them from traveling very far.
It’s no secret that this “first
light” has been around for quite some time, and it’s safe to say that it’s
a safe bet that the majority of the people who know about it will agree with
you. The Hubble constant is estimated to be 67.5 0.5 km/s/Mpc now based on
measurements of the minute fluctuations in this ancient radiation. Strangely, as both methods of measurement
have improved and gotten more precise, the gaps between the two estimates of
the Hubble constant have only widened. We call this discrepancy, and the
problems it causes, the “Hubble tension.” For cosmologists, this is a
major problem since it indicates that our knowledge of the fundamental physical
rules that control the cosmos is flawed.
Cepheid variables pick a side
Even though the universe is so large,
Anderson showed why even a change of a few km/s/Mpc in the Hubble constant is
significant. It’s been calculated that the observable universe is roughly
29,000 MPC wide. “This difference is extremely
important,” Anderson remarked. Let’s say you’re considering tunneling
through a mountain from opposite sides. If your calculations are accurate and
your interpretation of the rock type is correct, the two holes you are drilling
should intersect in the middle. But if they don’t, then either your estimates
are off or you’ve got the wrong kind of rock. According to Anderson, this is similar to
the Hubble tension and the current state of the Hubble constant.
He continued, “The more confirmation
we get that our calculations are accurate, the more we can conclude that the
discrepancy means our understanding of the universe is mistaken, that the
universe isn’t quite as we thought.” This technique now “takes a side”
in the Hubble tension dispute, agreeing with the “late time” option
thanks to the better calibration of the Cepheid variable measuring equipment. While Anderson’s team confirmed the 73
km/s/Mpc expansion rate, they also provided the most accurate calibrations of
Cepheids as distance measuring tools to date. Accordingly, “it means we
have to rethink the basic concepts that form the foundation of our overall understanding
of physics.”
The team’s findings have further
significance. Members of the research team noted, for instance, that a more
precise Cepheid calibration helps to better disclose the geometry of our
galaxy. Lead author and Ph.D. student in Anderson’s
lab Mauricio Cruz Reyes remarked, “Because our measurements are so
precise, they give us insight into the geometry of the Milky Way.” We can
now more precisely measure the Milky Way’s size and shape as a flat-disk
galaxy, as well as its distance from other galaxies, thanks to the extremely
precise calibration we established.