2. Dark Matter and the Large-Scale Universe

2. Dark Matter and the Large-Scale Universe

• Dark matter

Matter inferred from its gravitational effects, although it does not emit or absorb light like ordinary luminous matter.

• Luminous matter

Matter that emits, reflects, or otherwise interacts with light so it can be observed electromagnetically.

Doppler Effect and Galaxy Rotation

• Doppler effect

Motion changes observed wavelength. Toward us = blueshift; away from us = redshift.

• Astronomers measure Doppler shifts of stars and gas on different sides of a galaxy to determine how quickly they orbit its center.

• Rotation curve

Graph of orbital speed of stars or gas versus distance from a galaxy’s center.

• If most mass were concentrated where visible matter is, outer objects would be expected to orbit more slowly with increasing distance.

•Evidence

Many galaxies’ outer regions rotate faster than expected; their rotation curves stay roughly flat instead of dropping.

• Conclusion

Unexpectedly fast outer motion suggests additional unseen mass spread through an extended halo

Proposed Dark Matter Explanations

• MACHOs

Massive Astrophysical Compact Halo Objects—dim or nonluminous compact objects, such as faint stars or stellar remnants, that could contribute mass in a halo.

• WIMPs

Weakly Interacting Massive Particles—hypothetical particles with mass that interact very weakly with ordinary matter and light.

• Know what WIMPs are and recognize MACHOs and WIMPs as proposed explanations.

The Cosmic Web

• Cosmic web

Large-scale structure consisting of dark matter filaments, dense intersections, and large voids.

• Filaments

Long structures where dark matter and galaxies are distributed.

• Intersections

Dense meeting points of filaments where galaxies and galaxy clusters are commonly found.

• Voids

Large, relatively empty regions between filaments.

• Gravity and galaxy formation

Dark matter’s gravity gathers ordinary matter into dense regions; galaxies form and grow preferentially in these structures.