Question
Many of the scientific signs of the Quran has been proved by science such as the Big bang theory, the movement of moon etc..

Also I want to ask about the authenticity of the hadith in which Umar ibn al- Khattab said that he will cross question the angels in the grave regarding their religion and Creator. Is it possible that an angel who is assigned by God to ask certain faith-related questions be cross examined?

Answer
The noble sunna mentions the splitting of the moon during the time of the Prophet [pbuh], confirming the words of Allah Almighty in the Holy Qur`an:

The Hour (of Judgment) is nigh, and the moon is cleft asunder. [Al-Qamar: 1)

This incidence has reached us through a trustworthy chain of transmission from our master the Prophet [pbuh]. The splitting of the moon is a fact that cannot be contested by definitive rational and discernible proof. If, as the inquirer maintains, no discovery has been made yet to prove this, it does mean that it has not actually happened. The doors to scientific discoveries are open; perhaps sometime in the future, scientists will discover what they cannot prove today. We would like to inform the inquirer that some modern studies of the lunar surface suggest that the moon had indeed split—a fact that has led some people to embrace Islam when they realized that this event was mentioned in the Qur`an centuries ago.

Imam Abu al-Fida` Ismail Haqqi mentioned in Rawh al-Bayan Umar ibn al-Khattab's words about how he will be the one to question the angles in his grave (and not vice-versa). About this report, Al-Fatny wrote in Fitnat al-Mawdu'at, "It is an authentic expedient report." There is no legal deterrent to such a matter and Allah Almighty rewards His true believers with greater [steadfastness] out of his grace. Another similar report narrates how the angels were afraid to question Abu al-Ma'aly in his grave; he told them, "What is it you want? Are you the angels from my Lord? I spent my entire life making remembrance of Him and facilitating the way to His cause. What can you say when my words have spread everywhere and I used to be called Abu al-Ma'aly?!" They replied, "We know that you are Abu al-Ma'aly; rest in peace and do not heed [us]." After narrating this report, Imam Al-Suyuti wrote in Al-Hawy lil Fatawa: "[He was spared the angels' questioning] by virtue of his knowledge. It suffices him if this honor was the only reward for his knowledge."

A similar report is included by the hadith scholar Abu Al-Taher Al-Salafi in Al-Tuyuriyat through Sahl Ibn 'Ammar who said, "I saw Yazid Ibn Harun in a dream after his death and I asked him: "How did Allah deal with you?" He answered, "Two forbidding angels approached me in my grave and asked me: 'Who is your Lord? What is your religion? And who is your prophet?' So, I took my white beard in my hands and told them: 'Do you ask this of someone like me who taught the people the answer to your questions for over 80 years?! So, they departed".