Hubble Science
Expansion Rate / The Hubble Tension
When Hubble was launched one of its main objectives was to
measure the Hubble constant, the expansion rate of the universe.
Astronauts speaking on radio with ground control.
Beginning in the mid 2000s, around 2005, I started a project to use what are sort
of the gold standard tools in astronomy for measuring distances which is to use
pulsating stars called Cepheid variables, and exploding Stars called type 1A supernovae and
of course the Hubble Space Telescope itself. And to try to make more precise measurements
than had ever been made as a check on the universe. New observations from
the early history of the universe what's called The Cosmic Microwave Background,
we're beginning to make very precise predictions of how fast the universe ought to be expanding
today and so we wanted to follow up on that by making comparably precise measurements.
First it was the WMAP cosmic ray background satellite that NASA flew in the early 2000s.
And then that gave way to Planck, the European Space Agency satellite is even more precise.
So by measuring the cosmic microwave background and then using a model that we call the standard
model of cosmology to then extrapolate that to the present time they determined ultimately that
the universe ought to be expanding in funny units that we use 67.4 plus or minus 0.5 kilometers per
second per megaparsec, which roughly means the universe will double in about 10 billion years.
Using the Hubble Space Telescope and some of these tools the cepheid variables and the type 1A
supernovae we determined the local expansion rate to be about 73.0 plus or minus 1.0 kilometer per
second per megaparsec which is the most precise local or present measurement of the expansion rate.
But it differs from the expected value expected that is by the state of the universe shortly after
the big bang coupled with our understanding of the universe this cosmological model and
in fact those two now sit apart from each other by about five times their mutual error bar which
is a phenomenon we call now the Hubble Tension. To give you an analogy would be like if you had
a small child then you measured their height uh when they were two years old that would be like
the cosmic microwave background measurement, and then you used a model of how children
grow to predict how tall they ought to end up at adulthood and that would give you a height
and then you would actually measure when they grew up how tall they were. And so that's the comparison
we're making the present state of the measurement versus what is a very precise measurement in a
younger universe and then a model like the growth curve of a child to predict how tall they will be.
Except unlike a child we've seen many children grow we have a very good understanding of that
growth curve we've only ever seen one universe and it's full of stuff whose nature we don't deeply
understand and so it's not crazy to think that we might be missing something in that understanding.
In order to predict and really extrapolate the state of the universe from the beginning to the
present day we have to understand components of the universe, particularly two components whose
nature is not well understood about, makeup 96 percent of the universe and that's dark matter and dark
energy. Dark Energy makes up about 70 percent, and dark matter probably makes up about 25 to
27 percent. And we don't really understand at a detailed level what these are exactly we don't
understand they're microphysics so in order to make these predictions we assume that they are
their most vanilla or plainest possible forms. We see this tension then and so one possibility
not the only possibility is that they are more complicated that there's a more complex
story or some other aspect even that we've been missing about the universe. The Hubble
Space Telescope has more or less been working on measuring the Hubble constant for its entire
lifetime about 30 years so the original goal when it was launched was to measure it to 10 percent
uncertainty and I think that was successfully accomplished in the early 2000s. We're now on
sort of what I would say is the second generation of measurements of the Hubble constant that are
targeting closer to percent level precision and I think Hubble especially with its new instruments
has absolutely come through with the capabilities needed, Hubble really has delivered the quality
and caliber of data that's necessary to make these measurements.
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