Astronomy ch 29

Describe the evidence that the expansion of the universe is accelerating.

supernovae are fainter than would be expected if the universe were expanding at a constant rate.

Would acceleration of the universe occur if it were composed entirely of matter (that is, if there were no dark energy)?

there would be nothing about the matter that would "push things apart." In general, the gravity of regular matter tends to decelerate, not accelerate, the expansion.

Describe two properties of the universe that are not explained by the standard Big Bangmodel (without inflation). How does inflation explain these two properties?

does not explain why the mass-energy density of the universe would be equal to the critical density, nor does it explain the amazing uniformity of the universe. However, both of these features can be explained when an inflationary stage is added. with a rapid, early expansion stage

Describe at least two characteristics of the universe that are explained by the standard Big Bangmodel.

expansion, and the relative abundances of the very light elements (such as the existence of more helium than stars could have made since the Big Bang).

Which formed first: hydrogen nuclei or hydrogen atoms? Explain the sequence of events that led to each.

nuclei came first around 10-6 sec after Big Bang. Atoms did not come for 380,000 years later because the universe was not cool enough for electrons to capture protons.

Thinking about the ideas of space and time in Einstein's general theory of relativity, how do we explain the fact that all galaxies outside our Local Group show a redshift?

As the scale of space increases, waves of light wavelengths increase, which is a redshift. The farther away a galaxy is, the more space it travels through, and the greater the increase in its wavelength by the time it arrives.

Some theorists expected that observations would show that the density of matter in the universe is just equal to the critical density. Do the current observations support this hypothesis?

Every observation made to date shows that the density of matter is much less than the critical density.

There are a variety of ways of estimating the ages of various objects in the universe. Describe two of these ways, and indicate how well they agree with one another and with the age of the universe itself as estimated by its expansion.

The age of the universe can be estimated from the expansion rate. Ages in the solar system can be estimated by studying the radioactive decay of rocky material. ages of rock are only one third of expansion age

What are the basic observations about the universe that any theory of cosmology must explain?

Any cosmological theory must explain the expansion of the universe and the various stages the expansion has gone through cosmological principle: that on the large scale, the universe in any given time is the same everywhere (homogeneous and isotropic).

Astronomers have found that there is more helium in the universe than stars could have made in the 13.8 billion years that the universe has been in existence. How does the Big Bangscenario solve this problem?

At the temperatures and densities that existed between 3 and 4 minutes after the beginning, conditions (temperature and density) in the entire universe were right for protons and deuterium nuclei to fuse into helium. After 4 minutes, temperatures were not sufficient for the fusion of helium. so there was helium particles before stars that never became stars

Penzias and Wilson's discovery of the Cosmic Microwave Background (CMB) is a nice example of scientific serendipity—something that is found by chance but turns out to have a positive outcome. What were they looking for and what did they discover?

Penzias and Wilson were measuring the radio emission from a variety of astronomical sources, such as supernova remnants. They found radio static that turned out to be background radiation left over from early in the universe

Describe the anthropic principle. What are some properties of the universe that make it "ready" to have life forms like you in it?

The "anthropic principle"is the idea that the physical laws that we observe must be what they are precisely because these are the only physical laws that allow for the existence of humans. fluctuation that allowed for our galaxy conditions ballance of contraction to prevent universe from expanding and collapsing nuclear fusion rates that allow for suns to last a long time structure of atoms allowing for carbon to be released from suns

What are the advantages and disadvantages of using quasars to probe the early history of the universe?

Quasars are intrinsically very bright and can be seen at much larger distances than ordinary galaxies. Unfortunately, we don't know how the first quasars formed, so although clearly they formed very quickly after the Big Bang, we don't yet know why or how. Also, because the emission from the accretion disk around the black hole is so bright, we often cannot see the light from the dimmer galaxy that hosts the quasar. That means we don't really know how galaxies in the early universe influenced the evolution of quasars

What does the term Hubble time mean in cosmology, and what is the current best calculation for the Hubble time?

The Hubble time is the age of the universe, estimated time is about 14 to 15 billion years.

Describe some possible futures for the universethat scientists have come up with. What property of the universe determines which of these possibilities is the correct one?

expansion comes to a stop and reverses, with the universe ending in a "big crunch"—the implosion of matter, energy, space, and time. For this model, the mass-energy density r is greater than the critical density the universe continues to expand forever, but ever more slowly, coming to a stop only after infinite time. For this model, the mass-energy density is exactly equal to the critical density. the universe expands forever at a constant rate. This would occur in an empty universe. universe accelerates at a faster and faster rate forever. For this model, the mass-energy density is less than the critical energy density Current mass-energy density measurements indicate that the universe will expand forever.

What is dark energy and what evidence do astronomers have that it is an important component of the universe?

the expansion of the universe is speeding up. Such acceleration requires a source of energy. Scientists don't yet fully understand what dark energy is. It may be a new form of energy for which there is not yet a theoretical explanation. Alternately, it may be the vacuum energy associated with "empty" space itself, as predicted by quantum mechanics.

Suppose the universe expands forever. Describe what will become of the radiation from the primeval fireball. What will the future evolution of galaxies be like? Could life as we know it survive forever in such a universe? Why?

ultimately galaxies will consist of black holes, neutron stars, and white (and eventually black) dwarfs. As the latter two types of objects cool off, they will cease to radiate energy, and the galaxies will become cold and dark,

Why do astronomers believe there must be dark matter that is not in the form of atoms with protons and neutrons?

we see material in their outer regions moving around their centers too fast for the gravity we deduce from ordinary matter to hold. There must be some other form of material there with gravity. leading scientists to believe that this "dark matter" does not consist of ordinary particles, such as protons and neutrons.