The medical reason for "Goosebumps" are the caused by tiny little muscles flexing in the skin.  This causes the hair follicles to rise up and are a completely natural response caused by our own sympathetic nervous system.

TL;DR - Your hairs stand on end when your body instructed by your brain.

I remember reading an article many months ago about Goosebumps and it's related to the persons "fight or flight" body reaction.  What made this article stand out to me, was many many years ago when we were "hairier", this reaction would cause our body hair to stick up making ourselves appear much larger to predators - similar to that of a cat or a dog.

Our Bodies are the most advanced receptors available

Our bodies are great detectors of different types of energy, after reading a book about spells and witchcraft, they asked a few questions that rang very true amongst everybody I have ever spoken to, some of the questions were:

"Have you ever walked into a room and can feel there was an argument, or sense a bad atmosphere?"

"Have you ever thought to yourself, the phone will ring...only for it to ring a few seconds later?"

Although very difficult to prove with modern science, these are real life occurrences of our bodies being able to detect different types of energy, whether negative or positive.

Do you have an odd feeling that somebody is watching you?

I've lost track of how many times I've felt like somebody is stood over to the left of me, in fact, I think pretty much everybody can sense another presence, it could be in your office, it could be in your bedroom, or even a house you sometimes visit - we can always feel that something is "there".

I think about the time we went to a friends house, and after saying our goodbyes, both myself and my wife spoke about a certain area at the top of the stairs that felt like we weren't alone - how can 2 people have the same experience if there is no truth in our bodies being able to detect these odd energy patterns.

How can I detect Aliens with Goosebumps?

If you have read alien abduction stories, one common occurrence seems to prevail over all... "there was this electricity feeling" or "the air seemed charged", furthering the theory that our little Alien friends are generating some sort of electrical field when visiting us.

How to prove this theory?

Any UFO / Alien hunter wants is the ability to prove their findings and make a direct correlation between what they see/feel with some sort of scientific equipment that can be recorded for evidence.

We know many who often look for more information about Alien visitations do want the ability to prove to others they are not going crazy. Help yourself and start recording your feelings in a journal, and if possible use some sort of EMF detector to document the changes in electrical energy.