I was always under the impression that the first heaven includes this universe we are in but how can we understand it in terms of the ahadeeth about israa? I mean, in the ahadeeth the prophet sws is said to have had a gate to the first heaven opened for him where he met Adam 3alayhe salam. how can we understand this?
It can be derived from the Qur’an (Fusilat 41:12 and al-Mulk 67:5) that the sky we are seeing is indeed what the Qur’an refers to as “the lowest heaven,” which is an indication that this is the first heaven.
As for the Hadith of Isra and Miraj, regardless of how reliable these Ahadith are, please note that according to the Qur’an, the whole experience of the Miraj (ascenscion to heaven) of the Prophet (pbuh) was in fact a true dream (Ru’ya Sadiqa) – al-Isra 17:60. This of course does not mean that the experience was not real. Dreams are in fact one of the ways by which God reveals some of the hidden knowledge or wisdom to His prophets (pbut). However since the nature of these kinds of revelations is in the form of dream, they cannot be interpreted literally.
Even the text of these Ahadith indicate that they are narrating a dream. For instance there is a detailed explanation of people being punished in hell while we know that the punishment of hell only begins after the day of judgement (e.g. al-Takweer 81:12).
Therefore, when we read that the Prophet (pbuh) saw (in his dream) some of the prophets in different heavens this does not mean that these prophets were literally living in different heavens (which itself is a concept that is alien to the Qur’an). This can merely indicate the fact that each prophet has his own position in the eyes of the Almighty.