The third paragraph makes a transition from the introduction to the rest of the paper, telling us what they are going to discuss.Unfortunately, they only mention one source of gravity waves, while their paper goes on to discuss other sources and other topics as well.Without focus, papers tend not to flow well and are harder to read and understand.
Remember, there is no one correct way to say anything.
There is no one correct way to order or word your paper. And, in most cases, there is only one way to spell a given word!
A good one or two sentence thesis statement is needed before they end their introduction. "In this paper we will describe five types of gravitational wave sources: orbiting masses, coalescence, mass transfer, ...
, as well as several proposed methods of gravity wave detection."Notice how the descriptions of the various means of generating gravity waves are presented in a nice orderly way. This part of the paper flows very well --- you learn a lot of information without expending much time or effort to read or understand it.
As a result, it flows well --- it is easy and enjoyable to read. It could be a little more focused, and could use a stronger thesis statement. (It's not very flashy, and it wouldn't make a good advertisement for a TV show, but that's not the purpose of the abstract.) It clearly, concisely, and in order tells us what the paper will discuss.
It is obvious from the text that the authors have a fairly good understanding of the subject, and for the most part their science and reasoning is sound. If I were doing research and wanted to know specific things about a specific topic, this abstract would let me know right away if the paper contains what I'm looking for. Take a look at the first two "introduction" paragraphs.
Remember not to feel too bad if a fellow student, Writing Fellow, or professor has a large number of "negative" comments on a paper you think of as being nearly perfect. If you've never spent much time listening to the symphony, not only will you have a tough time playing classical music well, you probably will have a hard time even knowing if you sound well.
Since the purpose of your paper is to convey information to other people, other people's opinions of your work matter!
Have other's read your paper, and listen to their comments.
Accepting criticism is not easy, but it's the only way to learn to write.