I saw both of them on various books, so are both valid? Or is it just that 50% of the people are wrong?
I know it's "a universe", but I still have trouble with it. Is there a website that tells you if you should write "an" or "a" for any X word? It's really giving me trouble.
The initial sound of the word determines if it should be preceded by "a" or "an". Vowel sounds are preceded by "an"; consonants by "a".
In the case of "universe", the initial sound is /j/, not /u:/, so "a" is used.
"Historical" is a bit more complicated, as the sources I've checked only list pronunciations starting with /h/. I think, though, that uses of "an" are themselves chiefly historical, reflecting a time or dialect in which the "h" was pronounced so weakly as to make the word sound as though the initial sound was the vowel /i/.
To be safe, use "a historical".
Using 'an' before certain words starting with 'h', including 'hotel', 'historical', 'horrific', and 'heroic', while not incorrect, is nowadays considered old-fashioned by many people. Common in the 18th and 19th centuries, it survives in Britain among the educated members of the middle and upper classes, although it is dying out. It is recommended by The Times style guide ('prefer an hotel to a hotel, an historic to a historic, an heroic rather than a heroic'), and permitted by Fowler's Modern English Usage 3rd Edition (2000) - 'At the present time, especially in written English, there is abundant evidence for the use of an before habitual, historian, historic(al), horrific, and horrendous, but the choice of form remains open'.
I use 'an' myself before those words, but I am prepared to defend myself, As chepner says, to be safe, use 'a historical, hotel', etc.
Indefinite article before words starting with H
An before h-words
Other users have amply answered the question itself, but I thought I would chip in with a little explanation as to why. Some of this is likely familiar to you, but the whole thing may be of interest to others who manage to search out this question.
English, like many languages, abhors the hiatus. That is where two distinct vowel sounds occur in separate syllables, without a consonant in between them. It is important here to note that this is about sound, not letters; as @chepner noted, "universe" starts with consonant sound, like the 'y' in 'you'. There are different ways to avoid hiatus, such as interpolating an unwritten consonant (hence Noël, as written in French (meaning Christmas time), being pronounced in English at least as if it were written "nowell"). This is a form of epethesis. It can be done by contracting, as in the French c'est, which would be written uncontracted as ce est, but saying that as written would involve hiatus so it is avoided. It can be done by glottal stops, which happens a lot in dialects that drop consonants.
In the specific case of an indefinite article in English, hiatus is avoided by replacing the usual indefinite article, a, with the alternative an. A chicken, an egg. Universe and euphonium start with letters that are considered vowels, but the sound each starts with is a consonant sound, so they take a. In honest or honour, the sole initial consonant is silent, so I am an honest man, and Brutus is an honourable man.
The thing is, pronunciations in English vary hugely with dialect, and those have all shifted over time. Most dialects that are considered formal and polite (which is, of course, an arbitrary distinction) do not pronounce the 'h' on honest and honour, but do on history and hotel. By the rules that follow logically from the purpose of an, we can see that honest and honour should take an, but history and hotel take a.
The usual understanding is that, at the point where people started writing down these rules, the consonant-h was either missing or very weak on those other words, so we end up with a lot of printed texts that say "an historical", or "an hotel". Because they exist so much in text, and even in some older (or older-fashioned) grammar guides, they are still generally considered acceptable, and some fuddy-duddies still insist that they are more correct. Some people even make it sound less weird by dropping the 'h' from those words when they say them, but only when the word is preceded by the indefinite article. Thus "an historical" gets pronounced by some people as "an'istorical", even though they would say "this historical" with every consonant clearly enunciated as written.
As a final aside, this oddity of our definite article has caused some vocabulary to shift. I understand - though I may be wrong - that when we first imported oranges to Britain, we took the Spanish name for them: naranja (which actually itself came from Arabic). A person would say "a norange", shifting the other sounds to be more English-seeming. However, this being new to most people when they heard it, they did not know if it was "a norange" or "an orange", and the latter got standardised. The colour, of course, is named for the fruit.
It worked the other way for a while, before things settled down (though some dialects settled the other way) for 'uncle'. Back then, more words than just the indefinite article added an 'n' to avoid hiatus, so people would say "an uncle", "mine uncle", "thine uncle" - all of which could also be written "a nuncle", "my nuncle", "thy nuncle", all have the same meaning if you accept nuncle as having the same meaning as uncle, and for a time which was more used swung back and forth.
