# Hubble constant -- early vs recent nonagreement

There’s a problem in cosmology having to do with various measurements of the Hubble constant disagreeing. Basically, those that use measurments based on nearby things get around 74 km/sec/Mpc and those that use measurements from things in the early universe get about 67 km/sec/Mpc. Here’s a blog about it from the Bad Astronomer who explains things better than I can:

It’s currently thought that the universal expansion is accelerating. Cosmologists don’t really understand this, but to give it a name, they call it Dark Energy. Based on this, one would expect that the Hubble Constant in the early universe would be smaller than that of the current universe. Which is what they measure. So why is there a problem? Is the difference too big? If so, what should the difference be?

Or just maybe, all the measurements are correct and the universe is accelerating faster than current theories say.

Maybe I’m misunderstanding something, but since the early universe is also further away and therefore since “dark energy” is applied proportionately to how much space is between you and the other object it should be moving faster?

That’s embedded into the Hubble Constant. The constant is in the form of X km/sec/Mpc, where the last unit is Megaparsec. What it means is that for every Mpc the object is away, it’s going X km/sec faster. If it’s only 1 Mpc away, it’s moving X km/sec; if 2 Mpc away, 2X km/sec; etc.

My point is if they measure using only data derived from the earliest part of the universe, things weren’t expanding as fast then, so they should get a smaller Hubble Constant. The Constant is not really constant in the accelerating universe. But perhaps I’m not totally understanding it.

And in the early Universe, things also hadn’t been expanding for as long. In a universe without the cosmological constant/dark energy/quintessence/whatever the Heaven it is, the Hubble constant would be steadily decreasing with time. In a universe with no gravity, it’d be the inverse of the age, and in a universe with gravity (but none of the whatever-it-is), it’d be decreasing more rapidly than that.

Meanwhile, in a universe composed entirely of dark energy (or whatever), the Hubble Constant truly would be constant. That’s not a terrible approximation for our Universe, which is composed of about 70% dark energy (or whatever), but we do still have 30% of “normal” stuff, and the proportion was even higher in the past.