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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 amazing!
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
a 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.
Then again, 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 on
the Earth.
And in a lot of ways, being curious and exploring
those big questions is a big part of what
makes us human.
Thanks for watching this episode of SciShow
Space News, and thanks to all our patrons
on Patreon for helping us make it!
We wanted to give a special shout-out to this
week’s President of Space, SR Foxley.
Thanks for supporting us!
If you want to become our next President of
Space — or just help us keep making more
episodes of SciShow, you can head over to
patreon.com/scishow.
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