Can we measure acceleration of galaxies and stars?

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• elcaro
In summary, cosmologists and astrophysicists are not able to directly measure accelerations in most astrophysical systems. They can only measure the instantaneous velocity along the line of sight and rely on modeling to determine accelerations. This process can be complicated by biases and relies on assumptions about the average velocity distribution and the use of a virial model. If the mass of the system can be independently measured, deviations from the predicted velocities can suggest whether the system is expanding or collapsing.
elcaro
TL;DR Summary
We can (with increasing precission) measure distances of far away object in space, and velocities. But is there a method of measuring acceleration (so, in fact changes of velocity, or the time derivative of velocities) of far away galaxies.?
As for example we see a large void, the Great Repeller, which in fact is an underdense region, and with respect to this region, matter seems to be repelled by this region. The explenation for that is that matter outside that regions pulls on the matter inside it. But if that is really the explenation (attracation from surrounding matter, instead of repulsion from the center of that region) we would expect that closer to the center of that region, the acceleration falls down, whereas if there would be repulsion from this region, we would expect the accleration to increase when going further to the center. But can cosmologists/astrophysicist actually measure accelerations?

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The short answer is not for most astrophysical systems, not directly.

For the most part we can only accurately measure the instantaneous velocity along the line of sight. The rest comes from modeling. You assume some sort of average velocity distribution model for lots of objects, and then measure the line-of-sight velocities to fit the parameters of that model. If you get the model wrong, this can lead to biases in the imputed accelerations, which is one complication in interpreting data about galaxies and galaxy clusters.

A typical model that is used is a virial model: this one assumes that the object being observed is in a "steady state" configuration, and that its velocities can be described using the virial theorem. The virial theorem gives a relationship between the average kinetic and potential energy of a cloud of particles, and so by measuring their velocities, we get a measure of their kinetic energy, which tells us about their potential energy, which tells us about the gravitational field if the system is static.

If we have an independent way of measuring the mass of the system, such as gravitational lensing, then measuring higher velocities than predicted by the virial theorem may suggest the system is expanding over time. Lower velocities would mean the system is collapsing.

elcaro, ohwilleke, Bandersnatch and 1 other person

1. Can we measure the acceleration of individual galaxies and stars?

Yes, we can measure the acceleration of individual galaxies and stars through various techniques such as studying their motion and using gravitational lensing.

2. How do we measure the acceleration of galaxies and stars?

The most common method is through the study of their motion, specifically their velocity and distance from Earth. By tracking these properties over time, we can determine the acceleration of a galaxy or star.

3. What is the significance of measuring acceleration in galaxies and stars?

Measuring acceleration in galaxies and stars helps us understand the forces at play in the universe, such as gravity and dark energy. It also allows us to study the evolution and behavior of these celestial bodies.

4. Are there any limitations to measuring acceleration in galaxies and stars?

Yes, there are limitations such as observational errors and uncertainties in the data. Additionally, some galaxies and stars may have complex or unpredictable motion, making it difficult to accurately measure their acceleration.

5. How does measuring acceleration in galaxies and stars contribute to our understanding of the universe?

By measuring acceleration, we can better understand the structure and dynamics of the universe, including the formation and evolution of galaxies and the expansion of the universe. This information can also help us make predictions about the future of the universe.

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