WC has written before about the two mysteries confronting astrophysics: dark matter and dark energy. It turns out that visible matter – the stuff we can detect with existing astronomy equipment – accounts for only about 5% of the apparent mass of the universe. The other, missing mass is a mix of dark matter – estimated at about 27% – and dark energy – the other 68%. It’s embarrassing, if you are an astrophysicist. Where’s the other 95% of the universe?
The leading candidate was Weakly Interacting Massive Particles, WIMPs. The main theoretical characteristics of a WIMP were supposed to be:
- Interactions only through the weak nuclear force and gravity, or possibly other interactions with cross-sections no higher than the weak scale;
- Large mass compared to standard particles (WIMPs with sub-GeV masses may be considered to be light dark matter).
The problem is that after years of attempts at direct detection and indirect detection, no WIMPs have been found. WiMPs were a hypothesis; disproving a hypothesis is useful. But it leaves the missing mass still missing.
Now the focus has shifted to a second kind of hypothetical particle which might constitute the missing mass. The challenge is that this second possible particle, called an axion, is tiny even by atomic particle standards: less than a billionth of an electron’s mass.
To find something that only weakly interacts with ordinary matter and weighs something less than a billionth of an electron is a pretty good trick. The tool astrohysicists are using is called an ADMX. It’s a supercooled radio receiver surrounded by a strong magntic field. The theory of axions hold that they will interact with photons to create a radio wave. The challenge is that because the radio frequency this process generates is depndent on the axion’s mass, you have to listen to a lot offrequencies.
The other problem with axions is that they are so small that it would take a mind-boggling large number of them to account for the missing mass.
There’s even less progress, if that’s possible, on dark energy, the stuff that comprises two-thirds of the universe’s mass. There’s only indirect evidence of dark energy, albeit from three different sources. Dark energy accelerates the expansion of the universe. No one knows how.
it’s an exciting, maddening time in astrophysics.