May 25 th 1999 Using the HST, scientists were able to measure the distance to galaxies using pulsating stars called Cepheids (previous image) They measured the Hubble constant as 70km/sec/mpc This means that a galaxy appears to be moving 160,000 miles per hour faster for every 3.3 million light-years away
How did they do it? The team used the Hubble telescope to observe 18 galaxies out to 65 million light years. They discovered almost 800 Cepheid variable stars to measure the intergalactic distances reliably
How did they do it #2? V = H 0 d If a galaxy has been travelling at speed v for a time t since the Big Bang, the distance travelled is: d= vt Substituting this…. v/ H 0 =vt T = 1/ H 0 is the age of the universe
Why does it matter? This finding clears up the paradox from previous estimates: –How could the universe only be as old as the oldest stars at around 12 billion years…?
What now? The age estimate holds true if the universe is below the so-called critical density where it is delicately balanced between forever expanding and collapsing….? Open universe – keeps expanding Critical universe Closed universe- will eventually stop expanding and end up in Big Crunch Separation of 2 galaxies Time tBig crunch
Why should it stop expanding ? Slow expansion and gravitational attraction act in the opposite direction The final result depends on Ω Ω = ρ ÷ ρ c Where ρ = actual average density of universe and ρ c = critical density
So... What’s the value of Ω ? Estimates of ρ c are around 10 -26 kgm -3 It’s very uncertain due to the difficulties of measuring Hubble constant.. Highest estimates of ρ are of 10 -27 kgm -3 Therefore Ω ≈ 0.1 This looks like the Universe is open…
The 3 rd way ! Because cosmologists don’t like the idea of an open universe they decided that Ω = 1 which is a critical or flat universe The reason for them doing this is that they argue if the universe was going to expand forever or collapse we would have noticed by now….
Dark Matter In order to make Ω = 1 some more matter in the universe needed to be found Cosmologists called this matter Dark matter It is just hiding from us
Eh? Theories are: 1. It is baryonic matter (so it is so ordinary that it doesn’t affect absorbtion or emission of radiation 2. It’s non baryonic matter and we can’t detect it because we don’t know what we’re looking for