Aurich Lawson | Getty Images
Our Milky Way galaxy is hurtling through the emptiness of space at 600 kilometers per second, heading for something we can’t clearly see. The focal point of that 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.
Whisper in the air
As far back as the 1970s, astronomers noticed something strange about the galaxies in our nearby part of the universe. There was the usual and expected Hubble current, the general recession of galaxies driven by the overall expansion of the universe. But on top of that, there seemed to be a vague directionality, as if all the galaxies around us were moving to the same focal point as well.
Astronomers debated whether this was a real effect or an artifact of Malmquist bias, the bias we get in our observations because bright galaxies are easier to observe than faint galaxies (for fans of statistics, it’s just another expression of a selection -effect). It could be that a full count of the nearby cosmos, including the far more numerous small and faint galaxies, would wipe out any apparent extra movement and restore some sanity to the world.
But then came more detailed observations of the cosmic microwave background (CMB). The CMB is leftover light from when our Universe cooled from a plasma state and formed neutral atoms when it was only 380,000 years old — a relative baby compared to its current 13.77 billion-year existence. The CMB absolutely drenches the sky (and indeed the entire universe – something like 99.99 percent of all photons in the cosmos are part of the CMB), coming at us from every direction.
If I showed you a map of the CMB all over the sky, it wouldn’t look all that impressive – just a uniform blob of photons covering every square degree with a remarkably consistent temperature of about 2.75 Kelvin. But with enough sensitivity, you can detect a subtle difference of one in a thousand. The CMB is always slightly hotter in one direction on the air, 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 forward direction are blue-shifted to slightly higher energies, while photons coming from behind us are red-shifted to lower energies. Measuring the force of that shift reveals our total current speed – about 600 kilometers per second – and our direction: somewhere towards the constellation Centaurus.
We can easily explain part of that movement. The sun revolves around the center of the Milky Way galaxy and our galaxy itself is on a collision course with our closest neighbor, the Andromeda galaxy. Those combined movements account for part of the 600 km/s, but not all of it.
It seems that we – and almost all the galaxies around us – are heading to a random spot in the universe, forced to move against our will by a distant and unknown source of immense gravity.
The Great Attractor.
