Dark matter, dark energy and the expanding universe
Galaxies spin too fast and the expansion is speeding up. See the measurements that force both conclusions, what has already been ruled out, and exactly where the honest answer is still 'we don't know'.
THE FIRST CLUE
Galaxies spin the wrong way round
In the solar system, distance rules speed. Mercury races round at 47 km/s, the Earth at 30, Neptune at 5. That is exactly what gravity predicts when nearly all the mass sits in the middle. In the 1970s Vera Rubin and Kent Ford measured how fast stars orbit in other galaxies, expecting the same falling-off. They found that stars far out in the disc move just as fast as stars near the centre.
There is only one way that happens: a lot of mass must lie out where the stars are, and beyond them, spread through a huge invisible halo. Measure the light, and it is not there. Measure the gravity, and it is.
| What is orbiting | Distance from the centre | Speed |
|---|---|---|
| Earth, around the Sun | 150 million km | about 30 km/s |
| Neptune, around the Sun | 4.5 billion km | about 5 km/s |
| The Sun, around the galaxy | 26,000 light-years | about 220 km/s |
| A star twice as far out | 52,000 light-years | still about 220 km/s |
Check yourself
Pouya looks at that table and asks what the last row is really telling us. What is the strongest conclusion it supports?
- There is a great deal of mass out there that emits no light
- Stars far out in a galaxy have their own engines
- Gravity is weaker at large distances than we thought
- The distances to those stars were measured wrongly
Show the answer
There is a great deal of mass out there that emits no light
Right. Orbital speed depends on how much mass lies inside the orbit. A star at 52,000 light-years moving as fast as the Sun means there is far more mass inside its orbit than the visible stars can account for.
MORE WITNESSES
Three more measurements say the same thing
Rotation curves alone would be thin evidence. But in lesson 09 you saw that mass bends light. Weigh a galaxy cluster by how strongly it distorts the galaxies behind it, and you get about five times the mass its stars and gas can supply. The ripples in the microwave background from lesson 10 only come out the observed size if there is extra matter that does not interact with light. And simulations of the young universe only grow galaxies in the time available if that extra matter is there to pull gas together.
The strongest single case is a pair of galaxy clusters that collided. Their hot gas, which is most of the ordinary matter, smashed together and slowed in the middle, while the lensing shows most of the mass sailed straight through, along with the galaxies. Whatever the extra mass is, it barely interacts with anything, even with itself.
Check yourself
Dark matter is called dark because it is made of something black that absorbs light, like soot or dust.
Show the answer
False
False, and the name is unhelpful. Soot and dust absorb light, which makes them easy to spot: they dim and redden whatever is behind them, and they glow in infrared. Dark matter does none of that. It is transparent: light passes straight through as if it were not there. Its only known effect is gravity.
What has been ruled out, and what is left
Already excluded
Cold gas: would be seen in radio. Dust: would redden and dim starlight. Faint stars, planets and stray black holes: searches for their brief lensing flashes found far too few. And the recipe from lesson 10 sets a hard limit on how much ordinary matter of any kind exists: about 5% of the total, which is nowhere near enough.
What is left
Some kind of particle that has mass, moves slowly, and ignores light and ordinary matter almost completely. Nothing in the known list of particles fits. Underground detectors and particle colliders have been hunting for decades and have found nothing confirmed, which has ruled out many candidates without finding the answer.
Check yourself
For each statement about dark matter: supported, or not?
- It outweighs ordinary matter by roughly five to one
- Its presence is inferred from gravity in several independent ways
- We know which particle it is made of
- It is spread through a large halo around each galaxy
- It is mostly dead stars and cold planets
- It blocks the light of stars behind it
Show the answer
Supported by evidence: It outweighs ordinary matter by roughly five to one, Its presence is inferred from gravity in several independent ways, It is spread through a large halo around each galaxy
Not supported: We know which particle it is made of, It is mostly dead stars and cold planets, It blocks the light of stars behind it
THE SECOND MYSTERY
The expansion is speeding up
Everyone expected the expansion to be slowing down. Gravity pulls, and the only question was whether it would eventually stop the expansion or merely slow it for ever. In 1998 two competing teams measured the expansion rate far back in time and found the opposite: the expansion has been accelerating for the last several billion years. Something is pushing space apart faster and faster, and the name given to it, dark energy, is a label rather than an explanation.
The simplest description is a constant energy in every cubic metre of empty space, which does not dilute as space grows. That fits the data well. Why it should have the value it has is one of the largest unsolved problems in physics.
How we know: measuring the past with exploding white dwarfs
- Find a standard brightness
A white dwarf that pulls gas off a companion star gains mass until it crosses the 1.4 solar mass limit from lesson 05. Then it detonates completely. Because the trigger is always the same mass, these explosions all peak at close to the same true brightness.
- Use it as a ruler
If you know how bright something really is and you measure how bright it looks, you get its distance. Astronomers call an object like this a standard candle.
- Add the redshift
The same supernova's spectrum gives its redshift, which says how much the universe has expanded since the light left. Now you have distance and expansion, paired, for objects billions of light-years away.
- Read the result
Distant supernovae came out slightly fainter than a steadily slowing universe predicted, meaning they were further away than expected. Two independent teams got the same answer, and later work with the microwave background and with galaxy clustering agreed.
Check yourself
- A supernova with a small redshift, so fairly nearby, looks exactly as bright as a steadily slowing expansion predicts.
- A supernova with a large redshift, so far back in time, looks slightly fainter than that same model predicts.
What does the second case tell you, given that these explosions all have nearly the same true brightness?
- That supernova was a weaker explosion than the nearby one
- It is further away than a steadily slowing expansion would put it, so the expansion has sped up since
- The universe is younger than we thought
- Dust between us and it absorbed some of the light
Show the answer
It is further away than a steadily slowing expansion would put it, so the expansion has sped up since
Right. Same true brightness plus fainter appearance means greater distance. The light has been travelling through space that grew more than the old model allowed, and the two teams checked carefully that dust and changes in the explosions themselves could not account for it.
The budget, and what it hides
- Roughly 5% ordinary matter: every atom, star, planet and person. This is the part the first ten lessons were about.
- Roughly 27% dark matter: detected only through gravity, by four independent routes, composition unknown.
- Roughly 68% dark energy: inferred from accelerating expansion, nature completely unknown.
- The percentages are well measured. The names are placeholders. Knowing that 95% of the universe is something we cannot identify is a genuine result, not a failure, because it is a measurement rather than a guess.
Check yourself
Mahsa says dark matter and dark energy sound like excuses invented whenever the numbers do not work. What is the fairest reply?
- She is right: they are untestable labels invented to patch the equations, and scientists quietly admit it
- They are the same phenomenon under two names
- They are proven substances and the matter is closed
- They name measured gaps confirmed by independent methods, and rival ideas are still being tested
Show the answer
They name measured gaps confirmed by independent methods, and rival ideas are still being tested
Right, and the caution is healthy. Each was forced by measurement, each shows up in several unrelated observations, and each makes predictions that could fail. Modified gravity is taken seriously as an alternative: it does well on single galaxies but struggles with colliding clusters and the microwave background.
Lesson recap
- Stars at the edge of a galaxy orbit as fast as stars near the centre, which means a great deal of invisible mass is spread through a halo around it.
- Lensing, the microwave background and galaxy formation point to the same extra mass, and colliding clusters show it passes through almost everything.
- Ordinary matter of every kind is capped at about 5% of the universe, so dark matter is not dust, gas or dead stars.
- Type Ia supernovae act as standard candles, and they showed in 1998 that the expansion is accelerating. The cause is called dark energy.
- About 5% ordinary matter, 27% dark matter, 68% dark energy. The proportions are measured; the identities are not known.