# What is the reason we don't "see" the Universe's expansion?

• MBBphys
In summary, if you're trying to measure the expansion of space, you need to use very small units to make the measurement.
MBBphys
Gold Member

## Homework Statement

Ok, so if the universe is expanding, and the scale of space itself is changing, then that means that even humans and the Earth is expanding right?
But is the reason why we don't notice this effect the slowness of it?
So v=Hd, where H is Hubble's constant, which has a value of about 70 kms-1Mpc-1
So if we convert that to ms-1m-1-->it becomes (70000/(3.1x10^16))-->2.26 x 10^-12, that means a point one metre away from me is moving away from me at a speed of 2.26 x 10^-12 ms-1?
So if we take an entire human life to be, say, 100 years-->speed * time=2.26x10^-12 x 100 x 365 x 24 x 60 x 60=0.00713 metres-->is this why we do not "see" the expansion?

V=Hd

(See above)

MBBphys said:

## Homework Statement

Ok, so if the universe is expanding, and the scale of space itself is changing, then that means that even humans and the Earth is expanding right?
But is the reason why we don't notice this effect the slowness of it?
So v=Hd, where H is Hubble's constant, which has a value of about 70 kms-1Mpc-1
So if we convert that to ms-1m-1-->it becomes (70000/(3.1x10^16))-->2.26 x 10^-12, that means a point one metre away from me is moving away from me at a speed of 2.26 x 10^-12 ms-1?
So if we take an entire human life to be, say, 100 years-->speed * time=2.26x10^-12 x 100 x 365 x 24 x 60 x 60=0.00713 metres-->is this why we do not "see" the expansion?

V=Hd

## The Attempt at a Solution

(See above)
If our rulers are expanding at the same rate as we are expanding, how will we be able to measure any difference?

MBBphys said:

## Homework Statement

Ok, so if the universe is expanding, and the scale of space itself is changing, then that means that even humans and the Earth is expanding right?
But is the reason why we don't notice this effect the slowness of it?
So v=Hd, where H is Hubble's constant, which has a value of about 70 kms-1Mpc-1
So if we convert that to ms-1m-1-->it becomes (70000/(3.1x10^16))-->2.26 x 10^-12, that means a point one metre away from me is moving away from me at a speed of 2.26 x 10^-12 ms-1?
So if we take an entire human life to be, say, 100 years-->speed * time=2.26x10^-12 x 100 x 365 x 24 x 60 x 60=0.00713 metres-->is this why we do not "see" the expansion?

V=Hd

## The Attempt at a Solution

(See above)

I think your figures are a bit out. The lifetime expansion per meter is nearer ##6*10^{-9}m##. Which would be hard to detect.

A better example is the Earth's orbit round the Sun. The expansion of space would amount to about ##1km## in a lifetime. But, the Earth and the Sun are gravitationally bound, which means that gravity counteracts the minute expansion and keeps the two in the same orbit.

A more extreme example would be an object sitting on the ground. A slow expansion of space could not counteract gravity and lift it slowly off the ground.

Similarly the intermolecular forces that keep things together prevent you from expanding, even slowly!

## 1. What is the concept of the Universe's expansion?

The Universe's expansion is the idea that the space between galaxies is continuously growing, causing them to move further away from each other. This expansion is supported by the observation that galaxies that are farther away from us appear to be moving away at a faster rate.

## 2. Why is it difficult to "see" the Universe's expansion?

The main reason we cannot directly "see" the Universe's expansion is because it happens on a very large scale over a long period of time. This means that the changes are not easily noticeable to the human eye. Additionally, the expansion is happening in all directions, so it is difficult to observe from a single vantage point.

## 3. How do scientists know that the Universe is expanding?

Scientists have used a variety of methods to gather evidence for the Universe's expansion. One of the most prominent ways is through the observation of redshift in distant galaxies. Redshift occurs when the light from an object appears to shift towards the red end of the spectrum, indicating that the object is moving away from us. This has been observed in many galaxies, providing strong evidence for the Universe's expansion.

## 4. Is the Universe's expansion constant?

While the overall expansion of the Universe is generally accepted by scientists, there is still ongoing research and debate about the rate at which it is expanding. Some studies suggest that the expansion may be accelerating, while others propose that it is slowing down. More research is needed to fully understand the dynamics of the Universe's expansion.

## 5. How does the expansion of the Universe affect our daily lives?

On a day-to-day basis, the expansion of the Universe does not have a noticeable impact on our lives. However, it is an important concept in understanding the origins and future of the Universe. The expansion also has implications for the structure and evolution of galaxies and other celestial bodies. Additionally, it plays a role in the study of dark matter and dark energy, which are still largely unknown phenomena.

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