Midterm 2 Answers

A galaxy observed at redshift of z=9 has a prominent hydrogen spectral line. If this same spectral line has a wavelength of 200 nanometer (nm) when observed from hydrogen in a lab on earth, its observed wavelength from the galaxy is

2000 nm

The event horizon of a black hole with mass 1 M☉ has a radius of about

3 kilometer

How soon after the big bang did it become cool enough for neutral atoms to form?

a few hundred thousand years

In the first observation of gravitational waves in 2015, the object(s) created by the merger of two black holes is

a single black hole with slightly lower mass than the sum of the merging ones

Which of the following is a candidate for the identity of dark matter:

a. primordial black holes b. weakly interacting elementary particles (a and b)

Which of the following statements about dark matter is FALSE: a. it is observed to be a major part of the Milky Way b. it is observed to be present in other groups of galaxies c. astronomers have a pretty good idea what the dark matter is made of d. we can detect its gravity, even though we can't see it e. it may make up even more of the universe than the matter we can see

astronomers have a pretty good idea what the dark matter is made of

Today, we believe that only a small number of elements were actually formed during the big bang. Which of the following was NOT one of these: a. hydrogen b. carbon c. lithium d. helium e. all of the above were formed during the big bang

carbon

According to the theory of general relativity, the presence of mass

causes a curvature (or warping) of spacetime

The reason why big bang nucleosynthesis cannot occur earlier than about 1 minute is because

conditions must be sufficiently cool to not disrupt deuterium

Which of the following is not a prediction of General Relativity: a.perihelion advance of Mercury b.dark matter c.bending and lensing of light d.black holes e.gravitational waves

dark matter

Where in space did the expansion of the universe begin?

everywhere at once

When one member of a binary star system is a black hole, and astronomers detect flickering x-rays coming from the system, where are these x-rays usually coming from?

from the disk of material around the black hole (material that has been pulled from the companion star and is falling toward the black hole)

What do the surveys of the three-dimensional distribution of groups of galaxies reveal about how groups and clusters of galaxies are organized?

galaxy groups are organized in to huge filaments with great voids between them

The most prolifically created nuclear species in big bang nucleosynthesis is

helium 4, 4He

According to the Cosmological Principle, the universe

is isotropic and homogeneous

Time dilation means that

moving clocks run slow

We know that most heavy elements such as gold

neutron star mergers

The three pillars of evidence for big bang cosmology are the cosmological redshift, the cosmic microwave background, and

nucleosynthesis

Merging black holes can be observed

only by their gravitational wave emission

Galaxies we see today with a look- back time of 11 billion years or more, as compared to galaxies today, are generally

redder and smaller

The red and blue spots in images of the cosmic microwave background images represent

regions of small over- density and under- density

When astronauts aboard the International Space Station (ISS) in space let go of an orange, it just floats there. Why is that?

the ISS is falling around the Earth, and in free fall, things feel no weight

Astronomical observations such as galactic rotation curves provide evidence for dark matter in the universe. What do astronomers currently think the amount of visible matter in the universe compared to the amount of dark matter?

the amount of visible matter is much less than the amount of dark matter

Gravitational wave detectors can locate the position of an astrophysical object like a binary neutron star merger using

the difference in time of arrival of the waves at the different detectors

The principle of equivalence says that

the effects of gravity are equivalent to the effects of acceleration

The redshift observed in the light emitted by distant galaxies is due to

the expansion of space

If a very distant galaxy looks blue overall to astronomers, from this they can conclude that:

the galaxy must have a lot of hot, young stars

With enormous effort, a team of astronomers manages to collect enough light from a galaxy far, far away to produce a spectrum. That spectrum has lines from the elements carbon, silicon, and sulfur. This tells the team that

the galaxy must have had an entire generation of stars that was born, lived, and died

A gravitational lens is formed by

the mass between the source and earth

The first time that astronomers observed both gravitational waves and electromagnetic from the same event, what they were observing was:

the merger of two neutron stars

The main postulate of special relativity is that

the speed of light is an absolute constant

After the big bang, in order for the universe to become transparent to light and other electromagnetic radiation, what had to happen?

the universe had to cool enough for neutral hydrogen to form

The elementary particles making up ordinary matter on Earth today

up quark, down quark, electron

Redshift measurements of early galaxies indicate that galaxies and stars had formed

within 500 million years of the big bang