# Cosmology Question: Expansion of the Universe

• Moneer81
It's just a matter of finding the right values for the variables and plugging them into the equation. In summary, the conversation discusses whether smaller objects like a solar system, a star or a galaxy expand with the rest of the universe and the calculation of the effect of that expansion on a human being. The problem involves finding the time it would take for a human being to expand by 1mm and the height needed for that expansion to occur in one year, using the Hubble parameter as a constant. The conversation also mentions the use of Hubble's law and the need to find the right values for the variables in order to solve the problem.
Moneer81

## Homework Statement

So the other day in my cosmology class we talked about whether smaller objects like a solar system, a star or a galaxy expand with the rest of the universe. While the answer is obviously no, since local gravitational forces can overcome that expansion, our professor thought it would be kinda amusing the calculate the effect of that expansion on an object like a human being if we were to expand along with the universe.

So we're asked to find out how long it would it take us to expand by 1mm, given the current value of the Hubble parameter of Ho = 72km/s/Mpc, and assuming for the sake of this problem that the Hubble parameter is constant even if the time we find is very large.

We're also have to find how tall we have to be for that expansion to take one year.

I know it looks like a silly problem but I am just having a hard time finding a place to start thinking about it.

Thanks for taking the time.

Not takers?

I'm not a cosmologist but I'll take a complete shot in the dark at it.

It seems like the Hubble parameter value means that something which is one megaparsec in size will grow to become one megaparsec and 72km in size after one second. Is that right?

If that's true it seems like you'd have to rig up an algebraic equality with ratios, the

$$\frac{a}{b} = \frac{c}{d}$$

kinda thing, or an expression with calculus to account for compound growth

Hope I'm not misunderstanding and ridiculously oversimplifying the problem.

Moneer81 said:

## Homework Statement

So the other day in my cosmology class we talked about whether smaller objects like a solar system, a star or a galaxy expand with the rest of the universe. While the answer is obviously no, since local gravitational forces can overcome that expansion, our professor thought it would be kinda amusing the calculate the effect of that expansion on an object like a human being if we were to expand along with the universe.

So we're asked to find out how long it would it take us to expand by 1mm, given the current value of the Hubble parameter of Ho = 72km/s/Mpc, and assuming for the sake of this problem that the Hubble parameter is constant even if the time we find is very large.

We're also have to find how tall we have to be for that expansion to take one year.

I know it looks like a silly problem but I am just having a hard time finding a place to start thinking about it.

Thanks for taking the time.

Hubble's law is $$v= \frac{\Delta x}{\Delta t} = H_0 d$$

So you simply need to isolate delta x. This gives by how much the distance between two objects which were initially a distance "d" apart will have increased after a time delta t. (This assumes of course that delta t is not so large that H_0 will have changed value appreciably during that time interval)

CaptainQuasar said:
I'm not a cosmologist but I'll take a complete shot in the dark at it.

It seems like the Hubble parameter value means that something which is one megaparsec in size will grow to become one megaparsec and 72km in size after one second. Is that right?

If that's true it seems like you'd have to rig up an algebraic equality with ratios, the

$$\frac{a}{b} = \frac{c}{d}$$

kinda thing, or an expression with calculus to account for compound growth

Hope I'm not misunderstanding and ridiculously oversimplifying the problem.

The logic is correct. And it gives the same equation that I provided in my post.

## 1. What is the expansion of the universe?

The expansion of the universe refers to the phenomenon of the universe growing larger over time. This means that the distance between galaxies and other celestial bodies is increasing as the universe continues to expand.

## 2. What is causing the expansion of the universe?

The expansion of the universe is caused by a mysterious force called dark energy. This force is thought to make up about 70% of the total energy in the universe and is believed to be responsible for pushing galaxies apart.

## 3. How do scientists measure the expansion of the universe?

Scientists use a variety of methods to measure the expansion of the universe, including observing the redshift of light from distant galaxies, studying the cosmic microwave background radiation, and using standard candles such as type Ia supernovae. These methods allow scientists to track the changes in the universe's expansion over time.

## 4. Is the expansion of the universe accelerating?

Yes, current evidence suggests that the expansion of the universe is accelerating. This means that the rate at which the universe is expanding is increasing over time, which is one of the key pieces of evidence for the existence of dark energy.

## 5. What does the expansion of the universe mean for the future of the universe?

The expansion of the universe will continue indefinitely, and it is thought that eventually, the universe will reach a state of maximum entropy and become a cold, dark place. This is known as the "heat death" of the universe and is one of the possible outcomes predicted by current models of cosmology.

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