Wendy Freedman

the cosmic microwave background measurements have a precision of better than 1 percent.

they've really set the bar very, very high.

And that's just not possible yet to make measurements at that level of accuracy when you're trying to use stars that are millions out to hundreds of millions of light years away.

So far, there is no indication that the measurements themselves are an issue.

So the measurements from the Planck satellite, this European satellite, which is still the gold standard in the field, it's all sky.

And there are two groups on the ground, one in the Atacama Desert and one at the South Pole, that in fact came out with very recent measurements.

And they're very much in agreement.

The issue is in order to get the Hubble constant from those measurements, you have to have a model to fit the data.

So this is the beauty of this.

Given the model, you predict what the Hubble constant today should be.

How do you test the model?

You measure the Hubble constant today.

So if you can measure it with enough accuracy, not just precision, but accuracy.

So, you know, if you have a coin and you flip it, um,

If you do it a few times, you might get more heads than tails.

If you do it enough times and your coin isn't weighted in some funny way, it's going to come out 50-50.

And the more times you make the measurement, the more accurate your measurement is going to be.

But then there are other kinds of errors that no matter how many times you make your measurement, you're still going to have what we call systematic errors.

And an example of a systematic error would be

We know that stars like Cepheids form in the disc of galaxies where there's astrophysical dust.