Comet ice is black and in the darkness of interstellar space it is very black
Could dark matter just be black Interstellar comets?
Comet ice is black and interstellar space is cold, dark and vast. Star systems are only tiny specks in that cold vastness. What if there is a tiny sprinkling of ice out there between the stars?
Scientists have built enormous subterranean experiments to detect dark matter, the mysterious source of gravity that stops galaxies flying apart as they spin through space. These machines are looking for WIMPS (weakly interacting massive particles) that can travel though the Earth without noticing. But could there be a more boring source of this gravity?
What if Dark Matter was just building sized lumps of ice floating between the stars could we detect it?
Well surprisingly simple maths says we could not detect it.
The local dark matter density is about the mass of a third of proton per cubic metre.
That is 5.56×10-25 g/cm3 ( Tiny number: 5.56 divided by 10 twenty-five times)
The density of ice is 1 g/cm3
So to account for the mass of dark matter in interstellar space there only needs to be one volume of ice to every 1.8×1024 volumes of empty space.
For 10 metre building sized objects that is only one per 1.8×1027 cubic metres of space.
(Massive number 1.8 multiplied by 10 twenty-seven times) (lots of zeros on the end)
So we are talking about building sized icy objects 1.2 million kilometres apart.
If these objects are buzzing around through space the average distance they would travel before colliding with another lump of ice (mean free path) is 1.27×1025 metres or 1,342,419,000 light years. This means one of these objects would on average travel for tens of billions of years before colliding with another piece of ice.
How would we try and detect these objects?
What if we try pointing a telescope at a bright star watching for dark objects crossing on front of it? How far away could we see a black building sized object against a bright star? Lets guess at 10AU (ten times the distance of us from the Sun).
Black object crossing star
Detectable volume between us and star out to 10AU could be about 7×1014 cubic metres.
Probability of block of ice being in that volume = 7×1014 / 1.8×1027 =3.93×10-13.
So what would we expect to see? What does a probability of 3.93×10-13 mean?
Well, if these objects are slow-moving and take one second to cross the face of the star, we could expect an event every 1/(3.93e-13)=2.5445×1012 seconds or 80,632 years
Fast moving objects could only take a millisecond to flash across the star as we watch. Then we would only have to wait 80 years between events.
We might just stand a chance of spotting something if we could make telescopes that can monitor thousands of stars at a frame rates of thousands of frames per second. Remember normal telescopes use long exposure times far too slow for this type of work.