Dark Energy and the Expansion of the Universe

[punap18]

I am a high school senior taking an astronomy class this semester and we are currently doing a project where we ask a question that doesn't have a definite answer but we want to learn more about it. My question is: How does the universe continue to expand? Can anyone further explain this for me? What forces are causing this?

[bystander]

Start here: Dark Energy and the Metric Expansion of Space. The hyperlinks, references, footnotes, and external links should keep you busy for a while.

[Chris Peterson]

I am a high school senior taking an astronomy class this semester and we are currently doing a project where we ask a question that doesn't have a definite answer but we want to learn more about it. My question is: How does the universe continue to expand? Can anyone further explain this for me? What forces are causing this?

It is a good question, and bystander's links are a good start. But let me add one thing: you may be conflating a couple of different ideas here. As a high school senior, you should know that simple motion does not require any force. If you launch a rocket from the Earth at greater than escape velocity, it will move outwards from the Earth forever, despite the fact that the only force it experiences is the gravitational force between it and the Earth. That will cause it to continue slowing down, but it will never reach zero speed and start returning to Earth.

The same is true of the Universe. We long thought that the expansion was subject only to the force of gravity, and the question became one of whether its initial expansion was greater or less than escape velocity. That was the difference between everything ending in a Big Crunch or expanding forever into Heat Death. But with the recent evidence for dark energy, we now have an actual repulsive force, a sort of anti-gravity, that may be causing the Universe to expand ever faster as it gets larger (resulting in another final scenario, the Big Rip).

So be sure, in addressing your question, to separate expansion under the retarding force of gravity and expansion under the increasing force of dark energy.

[sallyseaver]

Start here: Dark Energy and the Metric Expansion of Space. The hyperlinks, references, footnotes, and external links should keep you busy for a while.

From the link that bystander provided for the "Metric Expansion of Space:"
"Metric expansion ... is modeled mathematically with the Friedmann-Lemaître-Robertson-Walker metric and is a generic property of the universe we inhabit." Here is a link to what a "metric" means in this context
https://en.wikipedia.org/wiki/Metric_(mathematics)
It is a mathematical formula for determining the distance between points (where a point is a unique location).

The usual metric used in distance equations for Newtonian mechanics is the Pythagorean formula. For example, in 2 dimensions, a distance between two points (x, y) and (a, b) is: (x-a)^2 + (y-b)^2
The Robertson-Walker metric has two additional terms. a(t), which can make the whole universe expand or shrink, and k, which changes the geometry of space.

It turns out that in astrophysics or space science that physical space is completely identified with the metric and coordinate system. The academic definition of space is: the set of all possible points (where points are thought of as a location with no distance or direction). [See definition of space at: http://www.einstein-online.info/diction ... e=en#space. Site provided by Max Planck Institute for Gravitational Physics, Golm/Potsdam]

With the expansion of space, the academic approach is to have a time-varying coordinate system, since the "space" is expanding. Given a starting coordinate system, there is a mathematical formula that enables calculations from one coordinate system to another.

From a course reference (Australian National University Edx course):
"One complication is that distance is not well defined in a curved expanding universe. Do we mean the distance now, or the distance when the light set out? Or the distance the light actually travelled? We can’t assume that flux (i.e. distribution of galaxies) follows the inverse square law, because the inverse square law was derived by assuming the photons were spread uniformly over a spherical surface of area which will not be true if space is curved. We’ve also ignored the redshifting of the photon energies and arrival rates. Luckily all these effects can be computed for a given cosmology."


So I think you will also want to give some attention to physical cosmology. "Physical cosmology is the study of the largest-scale structures and dynamics of the Universe."
https://en.wikipedia.org/wiki/Physical_cosmology

As you are already aware from the description of your project, "many things about the nature of dark energy remain matters of speculation" [Wikipedia article on Dark Energy]. This includes the source of a causal force.


In my research, physical space is a substance independent of the coordinate system. I bring this up as a contrast to the state of science knowledge that you are reporting on and grappling with (the standard accepted paradigm). Yes, math is needed to calculate distance, but in my paradigm, the coordinate system and metric are descriptive for a given problem, not intrinsically enmeshed in the definition of space itself.