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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