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I have read answers to several similar questions but I still don't get it. Earlier explanations seem to say that laws of quantum physics and general relative are different. Let me get this straight. As I understand things, planck length divided by planck time is the speed of light. Is that just a coincidence? If not, the speed of light, C, must be a constant ratio of Planck length/time. Correct? If only space expands, that would increase Planck length, which would change the ratio and thus the speed of light. Therefore, I am lead to believe that Planck length and Planck time must expand proportionally or the speed of light would not be constant. Where have I gone wrong? Please tell me why only space expands rather than spacetime.

Please don't overwhelm me, a novice, with complex equations that I can't handle yet. Many thanks.

Qmechanic
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Baird1939
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2 Answers2

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Planck's time results from a dimensional analysis involving the gravitational constant $G$, the speed of light $c$ and the reduced Planck constant $\hbar$, the result is

$$ t_p = \sqrt{\frac{\hbar G}{c^5}} $$

This is a constant of nature independent of expansion, that is $t_p$ had the same value 1Gyr ago, and 13 Gyr ago. Now you define Plank's length as

$$ \ell_p = c t_p $$

Again, this is a definition. If the universe expands, the ratio remains constant because $c$ remains constant

caverac
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Is it spacetime or space that is expanding?

Its both. But its better said that it is spacetime that is expanding and hence space is expanding too, since - obviously - space s a part of spacetime. The metric, the way we measure space and time remains the same (otherwise we would see no difference).

Dark energy is responsible for the expansion. It's essentially the energy carried by empty spacetime itself. It's the physical notion that corresponds to the cosmological constant term in Einsteins equation. Another way of thinking about it is that its a new force that only kicks in at the cosmological scale - we could call it the cosmological force to go along with the four forces - strong, weak, electromagnetic and gravitational.

Mozibur Ullah
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