Mystery of Cosmic Expansion Deepens
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We know the universe is expanding, but how fast? The more scientists explore this question, the more apparent it is that we still have a lot to learn about our universe.
Produced by the Space Telescope Science Institute’s Office of Public Outreach in collaboration with NASA’s Universe of Learning partners: Caltech/IPAC, NASA Jet Propulsion Laboratory, Smithsonian Astrophysical Observatory, and Sonoma State University.
- Hubble Space Telescope images of interacting galaxies: NASA, ESA, the Hubble Heritage Team (STScI, AURA), A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)
- Hubble Space Telescope images of interacting galaxies: NASA, ESA, the Hubble Heritage Team (STScI, AURA), A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)
- Animation, fly through deep field image: NASA, STscI; F. Summers, Z. Levay, L. Frattare, B. Mobasher, A. Koekemoer
- Animation, fly through galaxies: NASA/Dana Berry
- Cepheid variable RS Puppis pulse: NASA, STScI/H. Bond
- Image of Cosmic Microwave Background: ESA/Planck Collaboration
- Animation of Planck mapping the sky: ESA
- Illustration of Planck telescope: ESA
- Image of Hubble Space Telescope: NASA
- Hubble Space Telescope image of NGC 3368: NASA, ESA, and the LEGUS team
- Graphic comparing measurement results: STScI/L. Hustak
- Animation, zoom out from Andromeda galaxy: NASA, ESA, STScI/AURA, DSS
Written by Leah Ramsay
Designed by Leah Hustak
Editorial and design input from Timothy Rhue II
Subject-matter expertise provided by Dr. Adam Riess
Music from APM Music
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Where do we fit in all this?
Where do we fit in all this?
What is out there?
How does the universe work?
Mystery of cosmic expansion deepens.
NASA, STScI; F. Summers, Z. Levay, L. Fratture, B. Mobasher, A. Koekemoer.
For nearly 100 years, we have known that the universe is expanding
Space itself, the space between galaxies, is stretching
Much of our understanding of how the universe works, how it has changed over time and what the future may hold, is based on measurement of its expansion.
The big question is, how fast is the universe expanding?
There are several ways of measuring the expansion rate of the universe.
The mystery facing scientists is that the measurement methods are delivering very different answers
Cepheid Variable RS PUPPIS
One method for calculating the universe's expansion rate uses the light of stars that are close to us in space and time.
Using the Hubble Space Telescope, astronomers study the light of pulsing or exploding stars in nearby galaxies, measuring how quickly they are moving away from us.
Over many years, scientists have refined their measurements of these stars to more precisely calculate the expansion rate of the universe.
Another method for calculating the universe's expansion rate uses the oldest detectable light in the universe: the cosmic microwave background.
With the Planck Space Telescope, astronomers observed the light of the early universe and factored it into scientific models, predicting the expansion rate in our time.
Using the Planck Telescope and the cosmic microwave background, astronomers predict that the universe is now expanding at a rate of 67 kilometers per second per megaparsec.
Using the Hubble Telescope and stars in nearby galaxies, astronomers observe that the universe is expanding at a rate of 73 kilometers per second per megaparsec.
67 is not equal to 73
Scientists have arrived at these numbers after years of refining their measurements using both methods.
Animation showing how measurements have evolved between 2001 and 2019
The more evidence that is gathered, the more apparent the discrepancy becomes
Our observations of the universe no longer match our scientific predictions
For astronomers, not getting the answer you expect isn't a problem. It's exciting.
The different results from Planck and Hubble tell us that we are likely missing an important piece of the puzzle in understanding how the universe works.
Now we can look for it.
As the universe continues to expand, we continue our quest to understand how it works and our place within it.