the universe
List two methods of estimating the age of the universe.
(1) Measuring the Hubble constant and Hubble time of 1/H; (2) estimating based on the age of the oldest stars observed.
Identify and briefly describe the stages of an open and a flat universe according to the Big Bang theory if gravity acts alone.
(a) Big Bang explosion took place. (b) Galaxies formed. (c) Galaxies are still receding; the universe is expanding. (d) Original hydrogen will be used up; the resulting cold, black universe will continue expanding and decelerating indefinitely.
Briefly describe the stages of a closed universe according to the Big Bang theory if gravity acts alone.
(a) Big Bang occurred. (b) Galaxies formed and continued to recede. (c) We live in an expanding universe; galaxy clusters are racing away from one another today. (d) Galaxies will stop. (e) The universe will contract; galaxies will fall back inward. (f) Matter will be crunched together again.
Briefly describe the universe according to the Steady State theory.
(a) Galaxies are receding, the universe is expanding, new matter is being created, new galaxies are being formed. (b) The same pattern will occur. The universe maintains the same average density forever.
What are the best figures we have for the observable universe today (latest observations)? Age in approximately present form
13.7 ± 1% billion years old.
A cosmological constant
A cosmological constant dubbed dark energy, equivalent to a cosmic repulsion, introduced in General Relativity by Einstein, may drive an ever- accelerating expansion.
What are the best figures we have for the observable universe today (latest observations)? Approximate radius
Almost 14 billion light-years.
Why is it so important to measure the value of the Hubble constant very accurately?
An accurate value of the Hubble constant would be strong evidence in favor of one of the cosmological models described. The Hubble constant is used to figure the age and size of the universe.
cosmological models
Astronomers construct cosmological models, mathematical descriptions that try to explain how the universe began, how it is changing as time goes by, and what will happen to it in the future. These models must be consistent with the observational data we have on stars and galaxies.
cosmology
Cosmology is the study of the origin, present structure, evolution, and destiny of the universe.
a closed universe
If gravity acts alone, a closed universe, which began with the Big Bang, will not expand forever. Gravity will halt the expansion and force a collapse.
The Big Bang theory
If gravity acts alone, the open universe model says that the universe will continue to expand indefinitely. The flat universe model predicts a continu- ing but slowing expansion that approaches zero as time approaches infinity.
What did the discovery of the cosmic background radiation mean for the Steady State theory?
It invalidated the Steady State model, since the model cannot explain the existence of this radiation.
Massive neutrino
Massive neutrinos, MACHOs—massive compact halo objects—or WIMPs—weakly interacting massive particles—may exist.
cosmic background radiation
The Big Bang theory predicts that the universe should still be filled with cosmic background radiation, a vestige of the heat of its Big Bang origin.
The Hubble time
The Hubble time, the age of the universe since the time of the Big Bang, is equal to 1/H.
The critical density
The critical density is the minimum average density of matter and energy required to make the universe flat. The calculated value, which depends on the value of the Hubble constant (uncertain), is roughly 10-30 grams per cubic centimeter or a few hydrogen atoms per cubic meter.
What would be the cosmological significance of new discoveries of so-far unobserved dark matter and dark energy in the universe?
The density of matter in the universe would be closer to the critical value, evidencing that our universe is nearly flat.
The flatness problem
The flatness problem, how to explain why the earliest density of the universe must have been extraordinarily close to the critical density, is resolved by adding inflation theory to the Big Bang theory.
The oscillating universe
The oscillating universe model variation says that after the Big Crunch, another Big Bang will occur. Then a new expanding universe will be born out of the same matter. The universe oscillates forever.
Einstein-de Sitter model of inflationary cosmology
The preferred Einstein-de Sitter model of inflationary cosmology calls for a homogeneous, flat universe. Proponents hypothesize that an enormous amount of matter and energy in the universe is exotic and escaping detection.
What are the best figures we have for the observable universe today (latest observations)? (a) Rate of recession of distant galaxies (Hubble constant)
The rate of recession (Hubble constant) is 74 ± 5% km/sec/Mpc (23 km per second per million light-years).
The Hubble radius
The size of the observable universe is the distance that light has had time to travel since the Big Bang. The Hubble radius, equal to the speed of light divided by the Hubble constant, c/H, is about 14 billion light years. It increases as the universe ages.
When we look into space we see galaxy clusters receding from us. What does this observation imply about the universe?
The universe must be expanding. (Gravity binds galaxies within the clusters.)
TWO basic types of the models
Two basic types of models, evolutionary and steady state, have been tested in the last 50 years. Results substantiate the evolutionary type.
Summarize three observations that support the Big Bang model.
Your answer should include the following observations: (1) redshifts of distant galaxies (universe is expanding); (2) cosmic background radiation (remnant of heat from Big Bang); (3) abundances of hydrogen and helium (lightest elements formed in the first few minutes).
the hubble law
a galaxy's velocity of recession, v, is directly proportional to its distance from us, d. The formula is: v = Hd where H is called the Hubble constant. It gives the rate at which the galaxies are receding, or the rate at which the universe is expanding. It is also used in the Hubble law to figure the distance to galaxies from their measured redshifts. The best present-day value is Ho = 74 km/sec/Mpc (23 km/sec/MLY) ± 5% from independent measurements by the Hubble Space Telescope and satellites.
History of the universe from the Big Bang to the present.
big bang->inflation->expansion->present day acceleration
How can astronomers find out what the universe was like (a) 2 million years ago? (b) 3 billion years ago?
hey examine radiation emitted by distant galaxies such as (a) Andromeda, which is about 2.5 million light-years away from Earth, and (b) Hydra, which is about 3 billion light-years away. It takes light one year to travel a distance of 1 light-year. The light we now receive left Andromeda 2.5 million years ago or Hydra 3 billion years ago; it tells us now what the universe was like then.
inflation
inflation, a brief phase of incredibly rapid expansion shortly after the Big Bang, to account for the present vast extent of the universe and its uniformity.
cosmological redshift.
light from distant galaxies is shifted in wavelength toward the red end (longer wavelengths) of the spectrum. This phenomenon is called the cosmological redshift(z) which results from an expansion of space. As distances between galaxy clusters increase, traveling light waves are stretched. Greater redshifts correspond to more distant galaxies and earlier eras. The most distant, youngest galaxies we observe have the greatest redshifts. They are moving away fastest of all .
the cosmological principle
states that on a sufficiently large scale the universe is homogeneous and isotropic. At any given time, the distribution of matter is the same everywhere in space, and the universe looks the same in all directions. The cosmological principle is important because it lets us assume that the small portion of space we can see is truly representative of all the rest of the universe that we cannot see. It allows us to formulate a theory that explains the entire universe, including those parts we cannot observe.
Omega
the ratio of the average density of all matter observed by its radiation or gravitational effect to the critical value, is now less than 1. If the universe is flat as theorized, the rest must be made of 72 percent dark energy.
The Steady State theory
the universe does not evolve or change in time. There was no beginning in the past and there will be no end in the future. Past, present, future—the universe is the same forever. The Steady State theory assumes the perfect cosmological principle, which says that the universe is the same everywhere on the large scale, at all times. It maintains the same average density of matter forever. Astronomers reject the Steady State theory because it contradicts observations. The hypothesis that nuclear reactions in red giant stars produced chemical elements from carbon to iron and that heavier elements were made in supernovas, advanced to explain how they got here without a Big Bang, survives intact in evolutionary models.
