The problem with time travel to the future is energy conservation. For instance, let's say you create a time travel portal at the end of the universe, and then attach that portal to a trampoline several feet above a big red rug.
When time travelers try to touch the portal, they will experience time compression. If they stand on top of the rug, they will experience time expansion. And if they are in a vacuum, they will experience time contraction. The only way they'll be able to get from the portal back to normal reality is to stand on top of the rug, or go someplace where air is moving.
This makes sense, because, you see, when you travel back in time, you're traveling back into the future. When you travel forward in time, you're going backwards. So it is possible for someone in a vacuum to travel backwards, but it would require the use of a very large and very hot vacuum pump. In addition, since there isn't enough air for the entire trip back in time, it's likely that the traveler would experience more negative effects than positive. This could include nausea, vomiting, dizziness, and other problems that aren't related to gravity.
But if there is air in the space between the portal and the trampoline, the time travelers won't have to worry about the air pressure as much, because the portals don't create as much of an area where gravity is going on. Since air is less dense than vacuum, a bigger vacuum pump will be needed to push the air out of the way.
Also, in a vacuum, there isn't a need for a time machine, because time has no meaning there. So, if someone were in a place where time had meaning, they would have a choice. They can choose to travel back in time and choose a time in the future where they can live peacefully, or they can choose to travel back in time and choose a time in the future in which they can destroy all life in their present.
There's a problem with this, though. In order to destroy all life, they'd have to travel back to the beginning of time, right back to the Big Bang, before everything began, when the first particle was created. This sounds like a pretty big task, but it's not that difficult, since the Big Bang happened over 13 billion years ago.
To understand why this is so difficult, you have to understand what was going on at the Big Bang. At that point, the cosmos had a fairly stable form, and all the particles and forces we know today were already at work.
It's not that complicated to explain, really. All the elementary particles, like protons and neutrons, had been in place for a long time before the Big Bang. Then, as matter and antimatter began to vibrate, it caused the formation of a new particle called a Baryon, which is positively charged, and negatively charged, or neutral. When this particle combines with two protons and two neutrons, they create a proton-neutron pair.
Protons and neutrons are the building blocks of atoms. Because of the way atoms form, they have very little interaction with one another. The universe, then, is made up of neutrons and protons.
If people want to time travel to the past and destroy all the atoms in existence, it would take something much stronger than just hydrogen. – but in order to do it, the atom bomb would have to be bigger than hydrogen. Although hydrogen is light enough to create a small amount of radiation, it's very dense and would cause the universe to expand very quickly, which would make the radiation very high-energy.
Since the particles we know today didn't exist during the early universe, it's difficult to do time travel, and place your destination far in the past. without creating paradoxes. It may be possible to create them, but then the entire reality of time and space wouldn't exist, and you'd never be able to go back again.