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Chapter 26: Problem 12

. What does it mean to say that the universe is homogeneous? That it is isotropic?

Short Answer

Expert verified
The universe being homogeneous means it is uniform (the same) everywhere when looked at on a large scale. Being isotropic means the universe appears the same in all directions from any given point.

Step by step solution

01

Understanding Homogeneous

The term homogeneous in cosmology refers to the idea that the universe, when viewed at a sufficiently large scale, looks the same anywhere we look. This means that its composition and other physical properties are the same throughout.
02

Understanding Isotropic

Isotropic implies that the universe appears the same in all directions from any particular point. Therefore, whichever direction we look from a point, the universe will look the same.
03

Combining the Concepts

In summary, if the universe is both homogeneous and isotropic, which are fundamental presumptions in the standard Big Bang model, it implies that any observer, anywhere in the universe at any given point, will perceive the universe to be roughly the same in every direction.

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Most popular questions from this chapter

Use Starry Night Enthusiast \({ }^{\mathrm{TM}}\) to compare the distances of objects in the Tully Database with the radius of the Cosmic Light Horizon, the limit of our observable universe. As you will find, the most distant galaxies in this database are a long way away from the Earth and yet these distances are only a small fraction of the distances from which we can see light in our universe. Select Favourites \(>\) Deep Space \(>\) Tully Database to display this collection of galaxies in their correct 3-dimensional positions in space around our position. Stop Time and click on View \(>\) Feet to remove the image of the astronaut's suit from the view. Select Preferences from the File menu (Windows) or the Starry Night Enthusiast menu (Macintosh). In the Preferences dialog, select Cursor Tracking (HUD) in the drop-down box and ensure that Distance from observer, Name and Object type are selected. The view shows the boundaries of the Tully database as a cube. Use the location scroller (hold down the Shift key and mouse button while moving the mouse) to rotate the cube to allow you to choose galaxies on the outer fringes of this space. Use the Hand Tool to examine a selection of the furthest objects from the Earth, which is centered in the view, and write a list of \(10-20\) objects, noting the \(\mathbf{O b}\) ject type and Distance from observer. (a) In your sample, is there a predominance of any one kind of galaxy? If so, what type of galaxy appears to be most common at these distances? (b) Select the furthest of these galaxies and compare their distances with the radius of the cosmic light horizon. What fraction of the radius of the observable universe is covered by the Tully database?

What is a cosmological constant? Why did Einstein introduce it into cosmology?

The host galaxy of the supernova HST04Sas (see the image that opens this chapter) has a redshift \(z=1.390\). The light from this galaxy includes the Lyman-alpha \(\left(\mathrm{L}_{\alpha}\right)\) spectral line of hydrogen, with an unshifted wavelength of \(121.6 \mathrm{~nm}\). Calculate the wavelength at which we detect the Lyman-alpha photons from this galaxy. In what part of the electromagnetic spectrum does this wavelength lie?

Before the discovery of the cosmic microwave background, it seemed possible that we might be living in a "steady-state universe" with overall properties that do not change with time. The steady-state model, like the Big Bang model, assumes an expanding universe, but does not assume a "creation event." Instead, matter is assumed to be created continuously everywhere in space to ensure that the average density of the universe remains constant. Search the World Wide Web for information about the steady-state theory. Explain why the existence of the cosmic microwave background was a fatal blow to the steady- state theory.

Explain the difference between a Doppler shift and a cosmological redshift.
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