Short answer - because that is the nature of time. 

The long answer - I’m going to have to cover off a few things you might know so sorry if this is a bit duh, it’s a complicated question and the answer needs a to cover off a few things. 

As you are probably aware time and space are influenced by the presence of mass, mass slows the movement of time so less mass the more fluid. There was an experiment done where a matched pair of atomic clocks made, one taken up to high altitude and therefore the effect of gravity was lessened. The clock that stayed on earth kept its normal motion through time - relative to us. The clock that had been on the aircraft came back showing a difference. This is in essence time travel, where the earth-bound clock ages the clock on the airplane essentially went to the future, albeit a few fractions of a second - relative to the stationary one. Of course it didn’t time travel it just moved faster through time.

Then time isn’t fixed, like the universe that encloses it - it moves outward at different rates everywhere and where this not the case the sky would be blinding light because all the light every star had put out would still be there - rather than having moved away at roughly - the speed of light. 

So time is relative, not absolute but it still permeates all of the cosmos. 

Now to negate the effect of time - you have to slow it to the point you can reverse it right? And then to time travel you need to exceed its influence by speeding up past the speed of light. Which unfortunately is impossible because relativistic physics gets in the way. It’s all due to mass, light travels that fast - why can’t we? Because a light is made of photons and a photon has no mass, therefore it does not have to expend massive energy to accelerate like we do, for example you can speed particles to near light speed as they do in particle accelerators like the LHC but they can’t actually reach light speed because the particles they use have mass. It takes 4 TeV (4 terra volts) to power the experiment that they did to find the Higgs-boson. 

So then time can’t stop or flow backwards - unless you breach the light speed barrier. 

In theory if you could go faster than roughly 300,000kps then time would go backwards - relative to you of course. 

Another issue is the second law of thermodynamics which basically says that in a closed system - our universe appears to be a closed system - entropy can only increase. The direction time moves is forward - and this correlates with disorder and the direction of entropy. To introduce order or to force it we must expend energy and the first law of thermodynamics is that no machine is 100% efficient, you will lose energy through heat, friction and so on, increase force means increase in expenditure. Therefore no star trek or Dr How more to the point. 

So after all that crap basically time is a one way street, an effect of and intrinsic result of the big bang. Entropy is unstoppable in our enclosed system and time is a part of that. It’s also worth bearing in mind that time is something humans perceive in a particular way which we have evolved to do - like the ability to see in the visual spectrum and not say in the infra-red. Time is a directional force like expansion itself but not at all fixed.

But then there are tachyons which if they exist can move around past light speed and therefore time like it ain’t no thing. 

It doesn't have to be complex or particularly deep, and also it's not mandatory, but thanks for the stars and the company x

We are actually learning a huge amount about them now as astronomy is going through a sizable revolution in terms of data quantity and quality. They come in a vast variety as well so there is also that. One of the things that's been the most spectacular is that a facility in the USA and it's twin in Italy LIGO and VIRGO detected a phenomena that Einstein predicted called 'gravity waves' when two black holes collided a couple of years ago. This proved that when events like that happen the fabric of the universe ripples - and so do we more or less like an ocean wave - only we are the ocean!

You should also check out Neutron stars they are my favorite space wierdness here is an informative little vid - https://www.youtube.com/watch?v=udFxKZRyQt4&ab_channel=Kurzgesagt%E2%80%93InaNutshell

wow!!  Thank you for that!!

I find that mesmerising as well, I used to go out to a remote facility in the outback before the Corona to do research and in my off hours I'd lie on one of the out doors tables and watch the stars. 

Very cool question! 

Yes plus a few other factors, other than no one had ears at the time because there were no people, ears, time or atoms to make these things out of - at that stage.

However there is a hiss to the big bang it's a static that comes from the background radiation left over from the big bang. Inaudible unless you are a mechanical organism maybe. 

And the his is entirely measurable? What kind of things does it tell us?

Absolutely it's measurable, quite a long time ago the Hubble Telescope mapped it and there is a surprising relationship to it and the areas of high and low density in the cosmos, essentially if you turn the map inside out it's a snapshot of the ancient universe. 

Good gracious. That's pretty amazing. So the more there is the bigger the hiss then?

Can be done, if the texture match and neither is a feature, But there needs to be some padding - blue mainly - I'm thinking Prussian and maybe a yellow? 

Oh! Oh! Prussians! As in Germans! As in men who are sexy and nerdy at the same time! Oh! Oh!

Who dosen't love a hate fuck now and then? 

Are German men hateful when they fuck? Maybe I don't really want one then.

A hate fuck Scarly? So you do have a thing about Metal Heart then? HaHa.

I now apologise unreservedly.

Hahaha I'd break him I think 🤔 

If Preußischblau was just a couple of shades lighter It would be the most perfect color ever.

Yes. I rather feel that you would. HaHa.

You call it Preußischblau too? I thought that might be an English term. That would be a nice colour, I love light blues, I painted my living room a light cyan years ago it was like being in the sky. 

I don't think the Prussians call it Prussian Blue. But probably the rest of Germany does. My folk in Baden-Württemberg do. Its like French fries. I bet the French just called them Le Fri, before they turned on America and Freedon and President W and our fighting men abroad and Jesus. And the name had to be changed to save the Free World. I'm not saying Americans can be petty but .....

Cyan is nice. I like it with a light burgundy and white. (I don't mean wine there. It might look like I did).