Aurich Lawson | Getty Images
Our galaxy, the Milky Way, is hurtling through the vacuum of space at 600 kilometers per second, heading towards something we cannot clearly see. The focal point of this movement is the Great Attractor, the product of billions of years of cosmic evolution. But we will never reach our destination because, in a few billion years, the accelerating force of dark energy will tear the Universe apart.
whispers in the sky
As early as the 1970s, astronomers noticed that something funny was going on with the galaxies in our near part of the Universe. There was the usual and expected Hubble flux, the general recession of the galaxies driven by the global expansion of the Universe. But there seemed to be some vague directionality on top of that, as if all the galaxies near us were also heading towards the same focal point.
Astronomers have wondered if this is a real effect or an artifact of Malmquist bias, the bias we get in our observations because bright galaxies are easier to observe than dark ones (for stat fans, it’s just another expression of a selection effect). It could be that a full census of the nearby cosmos, including the much more numerous small and dim galaxies, will wipe out any apparent additional movement and restore some sense to the world.
But then came more detailed observations of the cosmic microwave background (CMB). CMB is the residual light from when our Universe cooled from a plasma state and formed neutral atoms when it was only 380,000 years old – a relative child compared to its 13, 77 billion years of current existence. The CMB absolutely permeates the sky (and, indeed, the entire universe – something like 99.99% of all photons in the cosmos are part of the CMB), coming from all directions.
If I were to show you a map of the CMB across the sky, it wouldn’t look so impressive – just a uniform blob of photons covering every square degree with a remarkably constant temperature of around 2.75 Kelvin. But with enough sensitivity, you can detect a subtle difference of one part in a thousand. The CMB is always slightly warmer in one direction of the sky, and it is equally cooler in the opposite direction.
NASA/WMAP Science Team
This is the CMB dipole, caused by the movement of the Earth through the Universe. Photons coming from the front are blue-shifted to slightly higher energies, while photons coming from behind us are red-shifted to lower energies. Measuring the force of this displacement reveals our current total speed – about 600 kilometers per second – and our direction: somewhere towards the constellation Centaurus.
We can easily explain part of this movement. The Sun is orbiting the center of the Milky Way galaxy, and our galaxy itself is heading on a collision course with our nearest neighbor, the Andromeda galaxy. These combined movements explain part of the 600 km/s, but not all of it.
It seems that we – and almost all the galaxies around us – are heading towards a random point in the Universe, forced to move against our will by a distant and unknown source of immense gravity.
The Great Attractor.