Since the moment it began, the universe has been expanding.
It took humanity a while to figure that out,
but over the last century, astronomers have gotten pretty good at calculating
how fast it’s happening and how
that speed has changed over the past 14 billion years.
Right now, there are two main methods for measuring this:
You can either observe astrophysical objects, like stars and supernovas,
or you can use the laws of physics to extrapolate
from data about the very old universe.
Both methods are great, but they also don’t quite agree.
And according to a new set of measurements to be published in The Astrophysical Journal,
that might not be a mistake.
The two numbers might actually be different.
And to explain that
we’d have to rethink our understanding of physics.
Right now, when we say that the universe is expanding,
we mostly mean that the void between the
galaxies and other large objects is growing.
It’s a technical thing, but strictly speaking,
the universe isn’t expanding everywhere.
Regardless, one of the tried and true methods of measuring this expansion requires calculating
the distances to stars called Cepheid variables.
A Cepheid is a star whose brightness changes over very regular periods of time.
And the length of that period is directly related to how bright the star is.
So as long as scientists can measure how fast these objects change,
they can figure out how bright they are up-close.
Then they can compare that number to
how bright the stars look from Earth
to determine their distance.
Using sets of Cepheids at different distances,
along with data about other kinds of objects,
you can then figure out how fast the universe is expanding.
There are a few other ways to measure this,
but Cepheid variables were especially important for this new study.
In it, researchers used the Hubble Space Telescope
to look at 70 Cepheids in a nearby dwarf galaxy:
the Large Magellanic Cloud.
It’s only about 162,000 light-years away,
which is super duper close on a universal scale.
Then, to make sure their brightness measurements were as accurate as possible,
the scientists combined their data with results from a few other sources,
including an international collaboration called the Araucaria Project.
This group calculated the distance to the Cloud a different way:
by watching the light of binary star systems
change as the stars moved around one another.
That movement allowed them to figure out stuff
like the stars’ masses and how big they are.
And by combining that with data about how fast those changes happened
and what kind of light the stars emitted,
the scientists could ultimately work out how far away they are.
After looking at all this data,
the authors of this new paper reported that
the universe is expanding at… drumroll please… about 74.03 kilometers per second per Megaparsec.
In other words, an object 1 million parsecs away
or roughly 3.3 million light-years
is moving away from us at about 74 kilometers per second.
An object 2 million parsecs away is
moving away at about 148 kilometers per second,
and so on and so forth.
74.03 kilometers per second per Megaparsec
That’s amazingly specific!
Now despite all the work that went into it,
that estimate isn’t actually groundbreaking at first glance,
since it’s basically in line with previous measurements.
But the key is that this number has far less uncertainty.
And that’s causing a problem,
because that estimate conflicts with other confident measurements about the universe’s expansion.
Like I mentioned earlier,
Cepheid variables aren’t the only way we can figure out how the universe is growing.
Another method is by studying the Cosmic Microwave Background, or CMB.
This is the oldest light in the universe that humanity will ever see.
It dates back to when the cosmos was only about 380,000 years old,
and studying it is the main objective of the European Space Agency’s Planck telescope.
By studying temperature fluctuations in this light,
scientists have been able to determine how fast
the universe was expanding those 13-ish billion years ago.
Then, they’ve been able to use that to extrapolate and figure out what the expansion rate should be today.
Those extrapolations are all based on, like,
really well-tested laws of physics, so you
would think these results would match up
pretty well with what we’ve observed with instruments like Hubble.
Except, that they don’t.
The Planck expansion rate is noticeably lower
than what we’ve gotten using sources like Cepheids.
It’s only 67.4 kilometers per second per Megaparsec.
This discrepancy isn’t new,
but there was always a chance that it was a fluke.
Like last year, scientists estimated that
there was a 1 in 3000 chance something had just gotten messed up.
But now, with this updated Hubble data,
the chance is 1 in 100,000.
Which means that while it’s not impossible
it is pretty unlikely these numbers are wrong.
In other words,
scientists now have to explain
why the observed expansion rate is almost
10% faster than what physics predicts it should be.
One current hypothesis is that
there was yet another incident where mysterious dark energy
caused an increase in the universe’s expansion rate.
Scientists don’t really know what dark energy is,
but they believe something like this has already happened twice
once for a brief moment after the Big Bang,
and again starting few billion years ago.
So maybe there was another incident like that between those two points.
Another idea is that dark matter interacts differently
with regular matter and light than we think.
Dark matter is stuff that doesn’t interact with light or charged particles,
so it’s basically invisible.
We only know it’s there
because of the gravitational effect it has on regular matter and light.
But we could be wrong about how strong its influence is on that stuff.
If its influence is stronger,
it could have countered the universe’s expansion early-on.
both of these ideas could also be wrong
maybe there’s some exotic particle we haven’t discovered yet that’s responsible for all of this.
Ultimately, this is yet another example of answers
in science just spurring more questions.
But there are ways scientists could explore this further,
including using gravitational waves produced in black hole and neutron star mergers.
Those are ripples in spacetime that
squish you know, like everything, like….
Everything that exists in space-time including earth
just a teeny bit as they travel through the cosmos.
Since they don’t rely on light,
measuring those waves would give us a totally new set
of data to study the expansion rate
but right now, this field of astronomy is really young,
so we can’t draw any conclusions.
In our day to day lives,
narrowing down these big-picture cosmological factors doesn’t
always feel that important.
Like, knowing how fast the universe is
expanding isn’t going to help you write a paper or
get through another day at work.
But this field is all about discovering and understanding the fundamental rules for how everything works
from Cepheids way out in space to the gravity that keeps you here on the planet.
And in a lot of ways,
being curious and exploring big questions
is a lot of what makes us human.
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Since the moment it began, the universe has been expanding.