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Hubble Science

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Expansion Rate / The Hubble Tension

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When Hubble was launched one of its
main objectives was to  

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measure the Hubble constant, the 
expansion rate of the universe.

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Astronauts speaking on radio with ground control.

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Beginning in the mid 2000s, around 2005, 
I started a project to use what are sort  

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of the gold standard tools in astronomy 
for measuring distances which is to use  

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pulsating stars called Cepheid variables, and 
exploding Stars called type 1A supernovae and  

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of course the Hubble Space Telescope itself. 
And to try to make more precise measurements  

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than had ever been made as a check on 
the universe. New observations from  

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the early history of the universe what's 
called The Cosmic Microwave Background,  

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we're beginning to make very precise predictions 
of how fast the universe ought to be expanding  

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today and so we wanted to follow up on that 
by making comparably precise measurements.  

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First it was the WMAP cosmic ray background 
satellite that NASA flew in the early 2000s.  

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And then that gave way to Planck, the European 
Space Agency satellite is even more precise.  

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So by measuring the cosmic microwave background 
and then using a model that we call the standard  

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model of cosmology to then extrapolate that to 
the present time they determined ultimately that  

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the universe ought to be expanding in funny units 
that we use 67.4 plus or minus 0.5 kilometers per  

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second per megaparsec, which roughly means the 
universe will double in about 10 billion years.

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Using the Hubble Space Telescope and some of 
these tools the cepheid variables and the type 1A  

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supernovae we determined the local expansion rate 
to be about 73.0 plus or minus 1.0 kilometer per  

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second per megaparsec which is the most precise 
local or present measurement of the expansion rate.  

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But it differs from the expected value expected 
that is by the state of the universe shortly after  

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the big bang coupled with our understanding 
of the universe this cosmological model and  

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in fact those two now sit apart from each other 
by about five times their mutual error bar which  

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is a phenomenon we call now the Hubble Tension. 
To give you an analogy would be like if you had  

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a small child then you measured their height uh 
when they were two years old that would be like  

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the cosmic microwave background measurement, 
and then you used a model of how children  

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grow to predict how tall they ought to end up 
at adulthood and that would give you a height  

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and then you would actually measure when they grew 
up how tall they were. And so that's the comparison  

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we're making the present state of the measurement 
versus what is a very precise measurement in a  

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younger universe and then a model like the growth 
curve of a child to predict how tall they will be.  

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Except unlike a child we've seen many children 
grow we have a very good understanding of that  

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growth curve we've only ever seen one universe and 
it's full of stuff whose nature we don't deeply  

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understand and so it's not crazy to think that we 
might be missing something in that understanding.

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

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present day we have to understand components of 
the universe, particularly two components whose  

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nature is not well understood about, makeup 96 percent
of the universe and that's dark matter and dark  

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energy. Dark Energy makes up about 70 percent, 
and dark matter probably makes up about 25 to  

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27 percent. And we don't really understand at a 
detailed level what these are exactly we don't  

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understand they're microphysics so in order to 
make these predictions we assume that they are  

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their most vanilla or plainest possible forms. 
We see this tension then and so one possibility  

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not the only possibility is that they are 
more complicated that there's a more complex  

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story or some other aspect even that we've been 
missing about the universe. The Hubble  

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Space Telescope has more or less been working 
on measuring the Hubble constant for its entire  

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lifetime about 30 years so the original goal when 
it was launched was to measure it to 10 percent  

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uncertainty and I think that was successfully 
accomplished in the early 2000s. We're now on  

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sort of what I would say is the second generation 
of measurements of the Hubble constant that are  

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targeting closer to percent level precision and I 
think Hubble especially with its new instruments  

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has absolutely come through with the capabilities 
needed, Hubble really has delivered the quality  

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and caliber of data that's necessary to 
make these measurements.

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