Question: > 
> He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. After making purification for sins, he sat down at the right hand of the Majesty on high (Hebrews 1:3 ESV)
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I am interested in the phrase ‘he upholds the universe’
My question is:- How could Christ uphold the universe as a helpless babe and also after he had died before the resurrection.

Answer: 
The simple answer is that He did not uphold the universe as a helpless baby. Hebrews 1:1-3 speaks of His deity/nature BEFORE He was born as a man with a human nature.

In fact, if you read the rest of Hebrews 1 you will discover His own Father acknowledges Jesus as God, vs8. Also at vs10, "And, Thou Lord in the beginning did lay the foundation of the earth and the heavens are the woks of Thy hands.

Moreover Luke 2:11, "for today in the city of David there has been born for you a Savior, WHO IS Christ the Lord." John 17:3,5, "And this is eternal life, that they may know Thee, the only true God, and Jesus Christ WHOM THOU HAS SENT." vs5, "And now, glorify Thou Me together with Thyself, Father, with the glory which I had with Thee BEFORE THE WORLD WAS."

Lastly, Philippians explains at verses 6-8 His pre-existence as God (And so does John 1:1-3) but emptied Himself, taking the form of a bond-servant/man, and being made in the likeness of men, And being found in appearance as a man, He humbled Himself by becoming obedient to the point of death, even death on a cross." And btw, God the Father and God the Holy Spirt did not go anywhere after Jesus died as a man before His resurrection. I think it's important to put things in perspective.
Read all 14 verses of Heb chapter 1, in its original Hebraic context and key terminologies, then ask WHO is the CREATOR and ABBA of the Son.

Hebrews 1:1-14
“ELOHIM, having in the past spoken to the fathers through the prophets at many times and in various ways,
1:2 has at the end of these days spoken to us by HIS Son, Whom HE appointed heir of all things, because of Whom also HE created the ages.
1:3 Who, being the radiance of HIS glory and the expression of HIS essence. And upholding all things by the power of His imrah, when He had by Himself made purification for our sins, sat down on the right hand of the MAJESTY on high;
1:4 having become so much better than the shamayim malachim - as He has inherited a more excellent name than they have.
1:5 For to which of the malachim did HE say at any time, You are MY son. Today have I begotten you; and again, I will be to him an ABBA, and he will be to ME a son? [Ps 2:7; 2Sa 7:14; 1Chr 17:13]
1:6 Moreover, when HE brings in the Firstborn into the world HE (WHO makes HIS malachim spirits and HIS servants flames of fire) says concerning the malachim,
1:7 Let all the “malachim of ELOHIM shachah Him. [Ps 104:4]
1:8 But of the Son HE says, Your throne, O Elohim, is forever and ever. You will rule Your Kingdom with the scepter of uprightness.518
1:9 You have loved righteousness and hated Torah-lessness; therefore ELOHIM, Your ELOHIM, has anointed You with the oil of gladness above Your fellows. [Ps 45:6-7]
1:10 And, YOU, ADONAY, in the beginning, laid the foundation of the earth. The shamayim are the works of YOUR hands.
1:11 They will perish, but YOU continue. They all will grow old like a garment does.
1:12 As a mantle, YOU will roll them up, and they will be transformed; but YOU are the same. YOUR years will not fail. [Ps 102:25-27]
1:13 Moreover, to which of the malachim has HE told at any time, Sit at MY right hand, until I make your enemies the footstool of your feet? [Ps 110:1]
1:14 Are not they all ministering spirits, sent out to do service for the sake of those who will inherit yeshuah?”

* extract from Book of The Covenant - An integrated translation & transliteration of the Hebraic scriptures.
Note: full caps refer to ADONAI-YHWH.