Measuring The Expansion Rate Of The Universe - Hubble Constant Tension Explained

2 months ago

Learn how Hubble is measuring the expansion rate of the Universe in this new explainer from NASA's Goddard Space Flight Center.

Credit: NASA's Goddard Space Flight Center
Producer & Director: James Leigh
Editor: Lucy Lund
Director of Photography: James Ball
Additional Editing & Photography: Matthew Duncan
Executive Producers: James Leigh & Matthew Duncan
Production & Post: Origin Films

Video Credit:
Hubble Space Telescope Animation
Credit: M. Kornmesser (ESA/Hubble)

Dark Energy Expansion Graph
Credit: NASA's Goddard Space Flight Center

Dark Energy Expansion Animation
Credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Hubble Extreme Deep Field Fly Through
Credit: NASA, ESA, and F. Summers, L. Frattare, T. Davis, Z. Levay, and G. Bacon (Viz3D Team, STScI)

James Webb Space Telescope Animations
Credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Music Credit:
“Alpha and Omega” by Laurent Parisi [SACEM] via KTSA Publishing [SACEM] and Universal Production Music
“Cosmic Call” by Immersive Music (Via Shutterstock Music)

Transcript

00:00When Hubble was launched, one of its main objectives was to measure the Hubble constant,

00:12the expansion rate of the universe.

00:27Beginning in the mid-2000s, around 2005, I started a project to use what are sort of

00:33the gold standard tools in astronomy for measuring distances, which is to use pulsating stars

00:40called Cepheid variables, and exploding stars called Type Ia supernovae, and of course the

00:45Hubble Space Telescope itself, and to try to make more precise measurements than had

00:49ever been made as a check on the universe.

00:53New observations from the early history of the universe, what's called the Cosmic Microwave

00:58Background, were beginning to make very precise predictions of how fast the universe ought

01:03to be expanding today, and so we wanted to follow up on that by making comparably precise

01:08measurements.

01:09First, it was the WMAP, Cosmic Microwave Background Satellite, that NASA flew in the early 2000s,

01:14and then that gave way to PLONK, the European Space Agency Satellite, that's even more precise.

01:20So by measuring the Cosmic Microwave Background, and then using a model that we call the Standard

01:26Model of Cosmology, to then extrapolate that to the present time, they determined ultimately

01:32that the universe ought to be expanding in funny units that we use, 67.4 plus or minus

01:38.5 kilometers per second per megaparsec, which roughly means the universe will double in

01:44about 10 billion years.

01:50Using the Hubble Space Telescope and some of these tools, the Cepheid Variables and

01:54the Type Ia Supernovae, we determined the local expansion rate to be about 73.0 plus

02:00or minus 1.0 kilometers per second per megaparsec, which is the most precise local or present

02:08measurement of the expansion rate.

02:10But it differs from the expected value, expected that is, by the state of the universe shortly

02:16after the Big Bang, coupled with our understanding of the universe, this cosmological model,

02:21and in fact those two now sit apart from each other by about five times their mutual error

02:26bar, which is a phenomenon we call now the Hubble Tension.

02:31To give you an analogy, it would be like if you had a small child and you measured their

02:35height when they were two years old, that would be like the Cosmic Microwave Background

02:40measurement, and then you used a model of how children grow to predict how tall they

02:45ought to end up at adulthood, and that would give you a height, and then you would actually

02:49measure when they grew up how tall they were.

02:52And so that's the comparison we're making, the present state of the measurement versus

02:56what is a very precise measurement in a younger universe, and then a model, like the growth

03:02curve of a child, to predict how tall they will be.

03:06Except unlike a child, we've seen many children grow, we have a very good understanding of

03:09that growth curve, but we've only ever seen one universe, and it's full of stuff whose

03:13nature we don't deeply understand, and so it's not crazy to think that we might be missing

03:19something in that understanding.

03:24In order to predict and really extrapolate the state of the universe from the beginning

03:28to the present day, we have to understand components of the universe, particularly two

03:33components whose nature is not well understood, but make up 96% of the universe, and that's

03:37dark matter and dark energy.

03:40Dark energy makes up about 70%, and dark matter probably makes up about 25 to 27%.

03:48And we don't really understand at a detailed level what these are exactly, we don't understand

03:52their microphysics.

03:54So in order to make these predictions, we assume that they are their most vanilla or

03:59plainest possible forms.

04:01We see this tension then, and so one possibility, not the only possibility, is that they are

04:06more complicated, that there's a more complex story, or some other aspect even that we've

04:11been missing about the universe.

04:16The Hubble Space Telescope has more or less been working on measuring the Hubble concept

04:20for its entire lifetime, about 30 years, so the original goal when it was launched was

04:25to measure it to 10% uncertainty, and I think that was successfully accomplished in the

04:29early 2000s.

04:31We're now on sort of what I would say is the second generation of measurements of the Hubble

04:35concept that are targeting closer to percent level precision, and I think Hubble, especially

04:41with its new instruments, has absolutely come through with the capabilities needed.

04:46Hubble really has delivered the quality and caliber of data that's necessary to make these

04:51measurements.

Measuring The Expansion Rate Of The Universe - Hubble Constant Tension Explained

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