Lectures 23 & 24: Dark Matter, Dark Energy, and Fate of the Universe

Dark Matter

An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence.

Dark Energy

An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate.

Mass Measurement Formula

Mass = (radius) × (velocity)²/G

Flat Rotation Curves of Spiral Galaxies

Spiral galaxies show flat rotation curves, indicating a large amount of invisible mass (dark matter).

Total Mass of Our Galaxy

Approximately ~10¹² MSun.

Stellar Mass of Our Galaxy

Approximately ~10¹¹ MSun.

Dark Matter in Spiral Galaxies

The visible portion of a galaxy lies deep in the heart of a large halo of dark matter.

Elliptical Galaxies

Stellar orbits in elliptical galaxies are randomly oriented, so no rotation curve measurement.

Width of Absorption Line

Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting.

Evidence for Dark Matter in Clusters of Galaxies

The mass we find from galaxy motions in a cluster is about 50 times larger than the mass we see in their stars.

Temperature of Hot Gas in Clusters

Temperature of hot gas (=particle motions) tells us cluster mass: 85% dark matter, 13% hot gas, 2% stars.

Gravitational Lensing

The bending of light rays by gravity, also tells us cluster's mass.

Doppler Shift

We can measure the velocities of galaxies in a cluster from their Doppler shifts.

3 methods of measuring cluster mass

Motions of cluster members galaxies (Doppler shifts), gas temperature in cluster (motions of gas particles), gravitational leasing. All indicate a similar amount of dark matter.

Gravitational Influence

The effect a mass exerts on other objects through gravity, controlling their motion, shape, and structure based on mass and distance.

Mass of Spiral Galaxy

The galaxy's mass increases with radius and becomes very large at large radius.

Blueshift and Redshift

One side of the galaxy shows blueshift while the other side shows redshift.

Gravitational Lensing Evidence

YES! We see gravitational lensing around a cluster of galaxies.

Galaxy Rotation Curves

We can measure rotation curves of galaxies (i.e., rotation velocities at various radii).

Motions of cluster member galaxies

A method of measuring cluster mass using Doppler shifts to analyze the velocities of galaxies within a cluster.

Gas temperature in cluster

A method of measuring cluster mass by observing the motions of gas particles and their temperature.

MACHOS

Massive Compact Halo Objects, which are dead or failed stars like white dwarfs and brown dwarfs that exist in the halos of galaxies.

WIMPS

Weakly Interacting Massive Particles, which are theoretical particles that are suggested to exist but have not yet been detected.

Gravitational Lensing Events

Occurrences where MACHOs make other stars appear brighter due to their gravitational influence.

Galaxy Formation

The process by which galaxies form, which is influenced by the presence of dark matter.

Protogalactic Clouds

Clouds of gas that contract under the influence of gravity, leading to the formation of galaxies.

Observational Constraints

Limitations based on observations that suggest there is not enough ordinary matter to account for the mass observed in the universe.

Theoretical Prediction

A hypothesis that WIMPs could be remnants from the Big Bang, contributing to the mass of the universe.

Energy Radiation

The process by which gas in protogalactic clouds loses energy by radiating light, allowing it to contract and condense.

Cluster Mass Measurement

The process of determining the mass of a cluster of galaxies using various methods such as galaxy velocities, gas temperature, and gravitational lensing.

Dark Stars

A hypothetical type of star that is not visible due to its dark matter composition.

Cluster Collision

An event where two clusters of galaxies collide, allowing for the observation of dark matter and gas interactions.

Gas Compression

The process that occurs when gas collides during a cluster collision, leading to increased density and temperature.

Dark Matter Existence

The hypothesis that dark matter exists based on observations that cannot be explained without it.

Galaxy Halo

The region surrounding a galaxy that contains dark matter and other celestial objects.

Invisible Stars

Stars that cannot be observed directly but are inferred to exist through their gravitational effects, such as MACHOs.

Big Bang

The leading explanation for the origin of the universe, which may have produced WIMPs as leftover particles.

Hubble's Law

A law that describes the expansion of the universe, stating that galaxies are moving away from us and that their speed is proportional to their distance.

Critical Density

The total mass in the current volume of the Universe required for it to be flat, where the current expansion speed equals the escape velocity.

Escape Velocity

The minimum speed needed for an object to break free from the gravitational attraction of a celestial body.

Large-Scale Structure

The distribution of galaxies and matter on scales larger than individual galaxies, including clusters, superclusters, and voids.

Supernovae

Explosive events that occur at the end of a star's life cycle, which can be used to measure distances in the universe.

Age of the Universe

Roughly estimated to be ~14 billion years, calculated as ~1/H0, where H0 is the Hubble constant.

Sloan Digital Sky Survey

A major astronomical survey that maps the positions of galaxies and helps reveal the large-scale structure of the universe.

Chains of Galaxies

Structures formed by the gravitational attraction of dark matter, leading to the alignment of galaxies in elongated formations.

Cosmological Principles

The assumptions that on very large scales, the universe is uniform and isotropic, with no center or edges.

Expansion of the Universe

The observation that galaxies are moving away from each other, suggesting that the universe is expanding over time.

Gravitational Contraction

The process by which matter is pulled together by gravity, leading to denser regions in the universe.

Models of the Universe

Theoretical frameworks that describe the structure and behavior of the universe, including the roles of dark matter and dark energy.

Brightness of Distant Supernovae

A method used to measure the expansion of the universe by observing the light output from supernovae.

Accelerating Universe

A state of the universe where the rate of expansion is increasing over time, as suggested by observations of distant supernovae.

Density Fluctuations

Variations in the density of matter in the early universe that led to the formation of large-scale structures.

Mass in the Universe

The total amount of matter, including dark matter and visible matter, that influences the gravitational dynamics of the universe.

Models of Dark Matter

Theoretical constructs that explain the behavior and distribution of dark matter in the universe.

Observations of Galaxy Maps

Empirical data collected from mapping the positions of galaxies, which supports theories of large-scale structure.

Expansion Speed

The rate at which the universe is expanding, which can be compared to escape velocity to determine the fate of the universe.