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  • { ♪ INTRO ♪ }

  • 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 atdrumroll pleaseabout 74.03 kilometers per second per Megaparsec.

  • In other words, an object 1 million parsecs

  • awayor 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 thatwhile it's not impossibleit 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 twiceonce 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 wrongmaybe 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 ratebut 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 worksfrom 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 Spaceor just help us keep making more

  • episodes of SciShow, you can head over to patreon.com/scishow.

  • { ♪ OUTRO ♪ }

{ ♪ INTRO ♪ }

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