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Cosmology > 7. Dark Matter > Flashcards

7. Dark Matter Flashcards

1
Q

How is the scale factor and the matter energy density related?

A

Energy density for matter is proportional to a^-3

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2
Q

What is the critical density?

A

The density of matter required to make the universe flat

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3
Q

Describe the properties of an elliptical galaxy

A

Old, little star formation

High M/L ~ 8 M solar / L solar visible

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4
Q

Describe the properties of a spiral galaxy

A

Young, lots of star formation

Low M/L ~ 0.3M solar / L solar visible

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5
Q

What is the density parameter of stars roughly

A

0.005 of the critical density

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6
Q

How much of the matter in the Universe is composed by stars?

A

A very small amount

- Most is between galaxies and baryonic matter is not visible at optical wavelengths

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7
Q

What is the density paramter for baryonic matter?

A

0.05 of the critical density

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8
Q

Why was dark matter hypothesised

A

There was not nearly enough matter to keep the universe flat from the critical density

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9
Q

What is dark matter?

A

Matter that has a gravitational influence, but does not react with light (emit/absorb/scatter)

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10
Q

What are the 3 pieces of evidence for dark matter?

A

Galaxy rotation curves
Galaxy cluser velocities
Gravitational lensing

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11
Q

What is the relationship of the rotational velocity and the distance from the centre of the galaxy?

A

Negative parabola
- As radius increases, enclose more mass until the edge of the galaxy
v = GM(R) / R

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12
Q

What does the data show about the relationship between the rotatational velocity and the radius from the centre of the galaxy and what does this infer?

A

Follows the negative parabola, but then stays constant afterwards
- Implies mass increases with radius beyond where it is visible as stars or M(R) is prop. to R

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13
Q

What is the mass/light ratio for dark matter?

A

60 M solar/ L solar (R_halo / 100 kpc)

- Considerably higher than the estimate for baryonic matter

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14
Q

How was dark matter hypothesised from the data of the velocity of galaxy clusters?

A

The velocities were way too high and they should fly apart unless there was extra matter

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15
Q

What is the Virial theorem?

A

For gravitationally bound objects in a cluster of size R_cl, W + 2K = 0

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16
Q

What is R_cl?

A

The estimated size of the cluster radius

e.g. half light radius

17
Q

What is the half light radius?

A

Add up all total light

Keep drawing bigger and bigger circles unti half the light is enclosed

18
Q

How can the mass be calculated from the Virial theorem, and what sort of result does it give?

A

M ~ < v > ^2 R_cl / G

  • Mass is summed mass over cluster
  • Includes mass in gas and DM between galaxies so is significantly higher
  • Density parameter ~ 0.2
19
Q

What is gravitational lensing, and how does it affect what we can see?

A

DM exerts a gravitational force and bends light

  • Background objects are magnified by lensing
  • Can detect very distant and old objects
20
Q

Describe weak and strong gravitational lensing

A

Weak - Distortion of images

Strong - Multiple images

21
Q

Is dark matter relativistic?

A

No it is “cold”

- Not moving fast enough to get caught into baryonic structure

22
Q

What are the two classes of DM?

A

MACHO - Massive compact halo objects

WIMP - Weakly interacting massive particle

23
Q

What are MACHOs?

A

Things that we know exist but can’t detect directly

  • Hypothesised to populate the halo of galaxies and explain DM
  • Bending causes magnification as light can take more than one path around the macho
24
Q

How does the macho change the observations of a background star

A

MACHO moves relative to background star

- brightness goes up and down meaning magnification is time dependent

25
Q

What is microlensing?

A

Small scale gravitational lensing

- Need LMC (large magellanic cloud) as its v. small

26
Q

How does the flux of an LMC subject to a MACHO change over time, and what can be concluded?

A

Bell curve shape for a flux-time graph

  • MACHOs been detected, but only 10% of the number needed to explain DM in our halo
  • DM more evenly distributed
27
Q

Have WIMPs been detected?

A

No

Cosmology (10 decks)

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