# Is Blue Shift just as provable as Red Shift?

I've heard a lot about red shift and it sounds pretty legit. But it made it sound as if blue shift was a lot less likely. Can anyone help me out?

## 2 Answers

The expansion of the Universe can be expressed using Hubble's law. This is that the recession velocity that a galaxy has with respect to our own is proportional to its distance from us. i.e. $$v = H_0 D,$$ where $v$ is the recession velocity, $D$ is the distance and H_0 is the Hubble "constant", which has a value of about 70 km/s per Megaparcsec.

Hubble's law is attributed tot he overall expansion of the universe and means that once we get further away than about 10 Mpc (or about 30 million light years), that other galaxies recede from us with velocities of 700 km/s or more. This recession produces a redshift.

However, superimposed on Hubble's law at a more local scale, individual galaxies are gravitationally interacting with each other and hence have their own peculiar motions with respect to the general "Hubble flow". In addition, don't forget our Sun rotates around the centre of our Galaxy with a velocity of some 230 km/s, so is heading in the direction of some galaxies and away from others. These local peculiar motions means that some nearby galaxies are moving towards us (at relatively modest speeds) and do exhibit a blue shift. An example would be the Andromeda galaxy at 2 million light years, which is approaching our Galaxy at about 110 km/s (or at a relative closing speed of 300 km/s with respect to the Sun).

But there are only about 50 galaxies in our local group and a further few tens of galaxies in some other structures out to about 30 million light years, and it is only from this small sample that any can exhibit blueshifts. The vast majority of known galaxies are further way and are swept along in the general universal expansion and hence away from us.

All galaxies other than those in the Local Group show red-shift (Doppler shift towards longer wavelengths which for optical frequencies is towards the red end of the spectrum, hence red-shift). This is due to the expansion of space from which we initially inferred the Big-Bang.

Galaxies within the Local Group and stars in the Milky Way (the Galaxy) may show either red or blue Doppler shifts depending on their motion with respect to the Earth.