What Are 10 Basic Issues That Still Puzzle Me About Modern Cosmology?

[mysearch]

As an interested hobbyist only, I have mainly been trying to understand some of the key ideas that underpin modern cosmology. While I may have made some `relative` progress, there are a number of really basic issues that still puzzle me. As it’s the New Year, I thought I start by trying to clear up what might be some obvious confusion on my part:

1. What caused the universe to expand for the 1st 7 billion years?

2. Why is Friedmann’s equation thought to define an expansive velocity as implicit within the Hubble constant (H)?

3. If the universe is homogeneous, at the very large scale, and for most of its existence has a very low energy density per unit volume of space, wouldn’t this approximate to a weak gravitational field model, with non-relativistic (local) speeds, and as a result the metric tensor of general relativity approaches the metric tensor of special relativity?

4. What additional information is GR telling us about the geometry within a large-scale model with no explicit centre of gravity, i.e. the point-mass implications of the Schwarzschild solution to Einstein’s field equations doesn’t seem to apply?

5. While accepting that mathematics can describe a 3D spatial curvature in terms of a higher dimension, is there any compelling data that suggests that spatial curvature [k] is not zero for all practical purposes?

6. Is there any sense that space is thought to have structure, which then explains how "matter tells space how to curve, and space tells matter how to move”, especially if (k=0)?

7. If (k=0), doesn’t the FLRW metric become a lot simpler with spacetime curvature essentially corresponding to the expansion of space, i.e. the path of 2 photons initially traveling in parallel will eventually be described by a geodesic due to the expansion of space rather than its spatial curvature. Is the metric suggesting more?

8. What is meant by the expansion of space that results in distance between galaxies increasing, i.e. if gravity is no longer described as a force, what geodesic curvature causes the galaxies to `roll away` from each other?

9. How is the increase in energy explained for any given volume of space in an expanding model with dark energy?

10. In a large-scale homogeneous model with no centre of gravity, does the concept of an overall change in gravitational potential energy, in an expanding universe, have any meaning?

That probably more than enough!

 

[zmike]

Apologies for the lengthy post.

Great questions! It sounds like you have done some research already, so I'm sure you will be able to find answers to your questions with some more digging.

1. The expansion of the universe during its first 7 billion years is believed to be due to the Big Bang, which is the theory that the universe began from an incredibly hot and dense state and then expanded rapidly.

2. Friedmann’s equation defines an expansive velocity as implicit within the Hubble constant (H) because it describes how the expansion rate of the universe changes over time. The equation also predicts that the universe will continue to expand as long as the density of matter and energy remains above a certain threshold.

3. Yes, if the universe is homogeneous and has a low energy density per unit volume of space, then it would approximate to a weak gravitational field model with non-relativistic (local) speeds. However, the metric tensor of general relativity would still differ from the metric tensor of special relativity, because general relativity accounts for the curvature of spacetime due to the presence of matter and energy, whereas special relativity does not.

4. General relativity provides information about the geometry of the universe even in the absence of a center of gravity. For example, it can be used to describe the shape of spacetime in the absence of a point mass, such as when describing the curvature of spacetime due to the presence of multiple masses or a uniform distribution of matter.

5. There is evidence that suggests spatial curvature [k] is not zero, such as measurements of the cosmic microwave background radiation and observations of distant supernova explosions. However, the current data is not conclusive, so there is still some debate among cosmologists as to whether or not the universe is spatially flat.

6. Space is thought to have some structure, but the exact nature of this structure is still unknown. The concept of "spacetime curvature" may help explain how matter tells space how to curve and how space tells matter how to move, even in the absence of a strong gravitational field.