AST Test #3 Part 2

Why is the law of surface brightness modified for redshifts?

A redshifted body that emits a thermal spectrum will still appear, but its temperature will appear lower

What are the properties of the cosmic microwave background: temperature, peak wavelength, degree of smoothness?

Peak wavelength: 1mm Temperature: 3000 K Degree of smoothness: 1/10,000

How do observations of the cosmic microwave background support the idea that the universe did indeed have a beginning?

Seeing that the universe originally emitted thermal radiation proves that it was indeed much denser at an early time. We are seeing back to very shortly after the Big Bang.

What contribution did Edwin Hubble make to our understanding of galaxies? How did his use of recognizable stars in "spiral nebulae" prove definitively that they were galaxies like our own Milky Way galaxy?

-He was the founder of extra-galactic astronomy -Knew we'd understand what spiral nebulae were if we knew how far away they were -He looked for recognizable stars within them. If spirals contained stars and if spirals were other galaxies, they would have to be far away. (M31 would be almost 1 Mpc, and it was probably closest.) *Other galaxies distances are much larger than the diameter of our galaxy*

Do galaxies look the same no matter how far away we find them, or can we look so far back in time as to see galaxies in the early stages of being formed?

According to the law of surface brightness: nearby and distant objects of the same kind have the same surface brightness.

How to determine the distance to a spiral nebula?

He looked for recognizable stars within them. If spirals contained stars and if spirals were other galaxies, they would have to be far away. (M31 would be almost 1 Mpc, and it was probably closest.)

What are the conditions that characterize the universe at very early times? Why do we know that, if the universe were dense, hot, and opaque at very early times, then it would have emitted a continuous spectrum of thermal radiation?

Hot and opaque and dense. This is because hot opaque objects emit blackbody radiation or thermal radiation. This causes a redshift which causes "cosmic stretching" or the stretching of light as the universe expands Remember that hot opaque objects have a very special form of spectrum (a continuous spectrum with a particular distribution of energy vs. wavelength.) This is called a thermal spectrum or blackbody spectrum, and it depends only on the temperature of the object.

How does Hubble's law indicate that the universe had a beginning in time, roughly 13.5 billion years ago?

Hubble's law states that the universe and milky way is expanding and the redshifts of distant galaxies are proportional to their distances from us *An expanding universe must have had a beginning, that is proven by the cosmic microwave background

How does the temperature of a star appear to shift due to the expansion of the universe? *Be able to use the formula that relates the observed temperature to the originally emitted temperature and to the expansion factor of the universe*.

In the expanding universe, distant objects also can have a large redshift. The redshift modifies the Law of Surface Brightness. A redshifted body that emits a thermal spectrum will still appear with a thermal spectrum, but its temperature will appear lower

How can we use knowledge of the temperature of the universe at the moment it switched from opaque to transparent, along with the expansion factor of the universe since that time, to predict the existence of the cosmic microwave background?

It causes a redshift. But, instead of thinking of this as a Doppler shift, it is better to think of this as a stretch that tracks the stretching of the whole universe. Expansion factor. The universe and its thermal radiation cooled as the universe expanded

How does the slow speed of light affect the appearance of the universe?

It is governed by the law of surface brightness. It's also an opportunity to be able to look at what the universe was like in the distant past by looking far away. Surface brightness- the amount of light an object gives off from any given square degree of its angular size

What did satellite measurements starting in the late 1980's reveal about departures of the cosmic microwave background from perfect smoothness, and why was that important?

The Cosmic Background Explorer (COBE) satellite was launched in 1989. It was the first to see the small fluctuations in brightness that indicated very slight variations in density of the universe. 1992: COBE team detects fine variations in cosmic microwave background Bumps" in the map are less than 1/10,000 of overall brightness.

What did Edwin Hubble discover when he plotted a graph of galaxy radial velocities (measured by Vesto Slipher) against the distances to those same galaxies that Hubble had himself measured?

The expansion of the universe is uniform

If you know the size of a galaxy (in parsecs or npc) and you measure its angular size, can you determine its distance?

Use: Angular size-distance formula to determine galaxy distances

What contribution did Vesto Melvin Slipher make to our knowledge of the universe? In what ways did the galaxy radial velocities that he measured lend support to the then -controversial idea that "spiral nebulae" were actually galaxies like our own Milky Way galaxy?

Vesto discovered that Doppler shifts gave their radical velocities and that galaxies are moving away very fast (faster than other cosmic objects) Also that almost all radial velocities had a positive sign which means the galaxies moving away from us. And that they were very large

How does the extreme smoothness of the cosmic microwave background support the idea that the universe has no center and no edges?

Without a center or an edge existing in the universe, every spot in the universe looks the same The extreme smoothness of the CMB confirms our belief in "no center, no edge" of the universe If you look far enough away in any direction, you see the moment when the universe became transparent.

Are we able to use the slowness of the speed of light (compared with astronomical distances) to "look back in time", by seeing parts of the universe so far away that the light left those galaxies early in the history of the universe?

Yes, because we can look at distant galaxies as they were in the distant past. (you can see early stages of galaxies if you look far enough away) Our view of the universe is necessarily a mixture of the nearby/recent and the distant/long-ago.