The Dark Side of the Universe - Professor Joseph Silk

TL;DR

Dark matter is unseen matter inferred from the gravity needed to explain galaxies’ motion, structure, and the universe’s mass budget. Professor Joseph Silk says ordinary matter accounts for only 5% of the universe, while dark matter is five times as abundant and may constitute 90% of the Milky Way’s mass. Astronomers test this picture by measuring stellar orbital speeds, making the evidence and search for direct detection worth exploring further.

Transcript

so good afternoon everybody um it's a pleasure to be here again to give my um next gesim lecture in the final one of this uh this Academic Year so I'm going to tackle today the uh the dark aspects of the universe the dark matter in the universe really um okay so we uh do have an issue here in in our understanding of the universe an awful lot of it ... Read More

Key Insights

• 🕶️ Dark matter was proposed in the 1930s to explain the expansion and structure of the universe.
• 🙌 Observations of rotation curves, gravitational lensing, and cosmic ray emissions support the existence of dark matter.
• 🕶️ The nature and composition of dark matter remain unknown, but theories such as WIMPs and supersymmetry provide potential explanations.

Questions & Answers

Q: What is dark matter, and why do scientists think it exists?

Dark matter is matter that cannot be seen directly but is inferred from its gravitational effects. Scientists think it is needed to explain the universe’s overall structure and mass budget, as well as the unexpectedly fast orbital motion of stars far from galactic centers.

Q: How much of the universe is ordinary matter compared with dark matter?

Professor Joseph Silk says ordinary matter, the material that makes up stars, planets, and people, is only 5% of the universe. He says there is five times as much dark matter, with an even larger component attributed to dark energy.

Q: How much of the Milky Way’s mass may be dark matter?

Silk says about 90% of the Milky Way’s mass is thought to be invisible. This unseen mass is proposed to occupy a dark halo surrounding the visible galaxy.

Q: How do astronomers measure dark matter in galaxies?

Astronomers measure the speeds of stars orbiting within a rotating galaxy. Because gravity controls orbital speed, those measurements reveal how much mass lies within the stars’ orbits, including mass that does not emit visible light.

Q: Why do stellar rotation speeds provide evidence for dark matter?

If visible matter were the whole mass of a galaxy, gravity should weaken and stellar orbital speeds should decrease farther from its center. Measurements of those speeds can instead indicate that additional unseen mass is supplying gravity.

Q: How did the expansion of the universe motivate the idea of dark matter?

In the 1920s, Hubble and George Lemaître identified the universe’s expansion through the recession of galaxies. Cosmologists then considered how matter’s attractive gravity counterbalanced expansion, leading them to suspect that much more matter existed than could be observed directly.

Q: What does the amount of matter imply about the universe’s future?

Silk explains that a universe with a great deal of matter could reach a maximum size and recollapse in a Big Crunch. With relatively little matter, it could continue expanding forever, so determining the total amount of matter became critical.

Q: Why is direct detection of dark matter still important?

Current evidence for dark matter is indirect, based on its apparent gravitational influence and its role in explaining cosmic structure. Silk says complete confidence will remain elusive until dark matter is actually detected directly.

Summary & Key Takeaways

• Dark matter is an invisible, mysterious substance that comprises the majority of the universe.

• The existence of dark matter was inferred to explain the expansion and future of the universe.

• Various scientific techniques, including gravitational lensing and particle accelerators, are used to study and detect dark matter.